I have an issue. I am making an extension class for a Collection and it is generic.. like
public static class ListExtensions
{
public static ICollection<T> Search<T>(this ICollection<T> collection, string stringToSearch)
{
ICollection<T> t1=null;
foreach (T t in collection)
{
Type k = t.GetType();
PropertyInfo pi = k.GetProperty("Name");
if (pi.GetValue(t,null).Equals(stringToSearch))
{
t1.Add(t);
}
}
return t1;
}
}
But I cannot add items to t1 as it is declared null.
Error: object reference not set to an instance of the object.
I am calling the method like
List<TestClass> listTC = new List<TestClass>();
listTC.Add(new TestClass { Name = "Ishu", Age = 21 });
listTC.Add(new TestClass { Name = "Vivek", Age = 40 });
listTC.Add(new TestClass { Name = "some one else", Age = 12 });
listTC.Search("Ishu");
And the test class is
public class TestClass
{
public string Name { get; set; }
public int Age { get; set; }
}
Using : (C#3.0) & Framework - 3.5
Thanks
As you probably don't want to manipulate (add, remove,...) the Search results after you performed a Search it is better practice to return IEnumerable<T> instead of ICollection<T>. Also C# has a special syntax for this: yield
public static class ListExtensions
{
public static IEnumerable<T> Search<T>(this ICollection<T> collection, string stringToSearch)
{
foreach (T t in collection)
{
Type k = t.GetType();
PropertyInfo pi = k.GetProperty("Name");
if (pi.GetValue(t,null).Equals(stringToSearch))
{
yield return t;
}
}
}
}
Well what kind of collection do you want to use? You've got to have an actual collection to add your results to. List<T> is probably the simplest suggestion. Just change the first line of the method:
ICollection<T> t1 = new List<T>();
EDIT: Although this is the simplest change to the code, you should definitely consider using an iterator block as per Thomas's answer.
Related
I want to build an expression for IQueryable GroupBy. While at the moment I'm just simplifying the problem to try and get it working, the eventual final implementation will involve the creation of quite complex expression trees so I want to build a complete expression that can then be integrated into other expressions.
I specifically want to build an expression of this overload:
public static System.Linq.IQueryable<TResult> GroupBy<TSource,TKey,TResult> (
this System.Linq.IQueryable<TSource> source,
System.Linq.Expressions.Expression<Func<TSource,TKey>> keySelector,
System.Linq.Expressions.Expression<Func<TKey,System.Collections.Generic.IEnumerable<TSource>,TResult>> resultSelector);
... my problem is in the implementation of the resultSelector and and the IEnumerable<TSource>.
I have a table of Customers (just dummy data for the purposes of working out this problem). This is stored in an SQL DB and I specifically want to use IQueryable to access the data.
public class Customer
{
public int Id { get; set; }
public string? FirstName { get; set; }
public string? LastName { get; set; }
public int Age { get; set; }
}
I also have a GroupResult class used to hold the results of the GroupBy (I have different constructors which I've been using in my testing to work out where my problem is occurring)
internal class GroupResult
{
public string? Name { get; set; }
public int NumRecords { get; set; }
public decimal AverageAge { get; set; }
public int TotalAge { get; set; }
public GroupResult() { }
public GroupResult(string name)
{
Name = name;
}
public GroupResult(IEnumerable<Customer> customers)
{
Name = Guid.NewGuid().ToString();
NumRecords = customers.Count();
}
public GroupResult(string name, IEnumerable<Customer> customers)
{
Name = name;
NumRecords = customers.Count();
}
}
The main static class that displays prompts to select column to group on, creates the relevant expression tree and executes it
internal static class SimpleGroupByCustomer
{
internal static DataContext db;
internal static void Execute()
{
using (db = new DataContext())
{
//get input
Console.WriteLine();
Console.WriteLine("Simple Customer GroupBy");
Console.WriteLine("=======================");
Console.WriteLine("Simple GroupBy on the Customer Table");
Console.WriteLine();
Console.WriteLine("Select the property that you want to group by.");
Console.WriteLine();
var dbSet = db.Set<Customer>();
var query = dbSet.AsQueryable();
//for this example we're just prompting for a column in the customer table
//GetColumnName is a helper function that lists the available columns and allows
//one to be selected
string colName = Wrapper.GetColumnName("Customer");
MethodInfo? method = typeof(SimpleGroupByCustomer).GetMethod("GetGroupBy",
BindingFlags.Static | BindingFlags.NonPublic);
if (method != null)
{
method = method.MakeGenericMethod(new Type[] { typeof(String), query.ElementType });
method.Invoke(null, new object[] { query, colName });
}
}
}
internal static void GetGroupBy<T, TTable>(IQueryable query, string colName)
{
Type TTmp = typeof(TTable);
var param = Expression.Parameter(TTmp, "c");
var prop = Expression.PropertyOrField(param, colName);
LambdaExpression keySelector = Expression.Lambda<Func<TTable, T>>(prop, param);
var param1 = Expression.Parameter(typeof(T), "Key");
var param2 = Expression.Parameter(typeof(IEnumerable<TTable>), "Customers");
var ci = typeof(GroupResult).GetConstructor(new[] { typeof(T), typeof(IEnumerable<TTable>) });
//var ci = typeof(GroupResult).GetConstructor(new[] { typeof(T) });
//var ci = typeof(GroupResult).GetConstructor(new[] { typeof(IEnumerable<TTable>) });
if (ci == null)
return;
var pExp = new ParameterExpression[] { param1, param2 };
var methodExpression = Expression.Lambda<Func<T, IEnumerable<TTable>, GroupResult>>(
Expression.New(ci, new Expression[] { param1, param2 }), //<--- ERROR HERE
pExp
);
Type[] typeArgs = new Type[] { typeof(TTable), typeof(T), typeof(GroupResult) };
Expression[] methodParams = new Expression[] { query.Expression, keySelector, methodExpression };
var resultExpression = Expression.Call(typeof(Queryable), "GroupBy", typeArgs, methodParams);
IQueryable dbQuery = query.Provider.CreateQuery(resultExpression);
if (dbQuery is IQueryable<GroupResult> results)
{
foreach (var result in results)
{
Console.WriteLine("{0,-15}\t{1}", result.Name, result.NumRecords.ToString());
}
}
}
}
When I run this and try and iterate through the results I get the following exception:
System.InvalidOperationException: 'variable 'Customers' of type 'System.Collections.Generic.IEnumerable`1[ExpressionTrees3.Data.Customer]' referenced from scope '', but it is not defined'
which is being caused by the param2 ParameterExpression marked above.
If I use the GroupResult constructor that just takes the key value
var ci = typeof(GroupResult).GetConstructor(new[] { typeof(T) });
and omit the param2 from the Lambda body definition the code works as expected and I get a collection of GroupResult records containing the distinct key values in the Name field (but obviously no summary value).
I've tried everything I can think of and just can't get past this error - it's as though the GroupBy is not actually producing the IEnumerable grouping of Customers for each key.
I suspect I'm missing something really obvious here, but just can't see it. Any help would really very much appreciated.
Please note that I am after answers to this specific issue, I'm not looking for alternative ways of doing a GroupBy (unless there's a fundamental reason why this shouldn't work) - this will be rolled into a much larger solution for building queries and I want to use the same process throughout.
Thanks Svyatoslav - as I thought, it was me being especially dumb!
Your comments, as well as a discussion with a friend who has a lot SQL knowledge pointed me in the right direction.
I had been thinking that the GroupBy expression was going to return an Enumerable for each key value and was trying to pass that into a function ... it always felt wrong, but I just ignored that and kept going.
It's obvious now that I need to tell the GroupBy what to calculate and return (i.e. your comment about aggregation).
So for this easy example, the solution is very simple:
var pExp = new ParameterExpression[] { param1, param2 };
var countTypes = new Type[] { typeof(TTable) };
var countParams = new Expression[] { param2 };
var countExp = Expression.Call(typeof(Enumerable), "Count", countTypes, countParams);
var methodExpression = Expression.Lambda<Func<T, IEnumerable<TTable>, GroupResult>>(
Expression.New(ci, new Expression[] { param1, countExp }),
pExp
);
Just by adding the 'Count' expression into the GroupBy method call it works!
.. and adding a new ctor for GroupResult:
public GroupResult(string name, int count)
{
Name = name;
NumRecords = count;
}
(yep, I feel a bit stupid!)
The question is similar to this one, but answer does not provide 2 critical things to me:
I need the code to work over navigational properties
I am trying to build extension method
I want to write queries like this:
this.context.User
.Where(t => t.Id > 10)
.SelectCustom(t => t.Address.Country.Title)
.OrderBy(t => t.DisplayName)
.Skip(10).Take(5);
With answer in provided link I get this far:
public class SelectList<TSource>
{
private List<MemberInfo> members = new List<MemberInfo>();
public SelectList<TSource> Add<TValue>(Expression<Func<TSource, TValue>> selector)
{
var member = ((MemberExpression)selector.Body).Member;
members.Add(member);
return this;
}
public Expression<Func<TSource, TResult>> ToDynamicColumns()
{
return this.members.??????????;
}
}
public static IQueryable<T> SelectCustom<T>(this IQueryable<T> query, Expression<Func<TSource, TKey>> FirstAdditional = null)
{
var columns = new SelectList<T>();
columns.Add(t => t.Id);
columns.Add(t => t.DisplayName)
if (FirstAdditional != null)
columns.Add(FirstAdditional);
return query.Select(columns.ToDynamicColumns);
}
Can this be done with EF Core 2.0?
EF will look through lambda invoke operations as if the body of that expression was inlined. So I would recommend leaving the source expressions alone and just generate expressions to invoke them.
Also I would keep the result type simple, and just return each row as an array of object. This should result in less overhead than creating lots of dictionaries. If you do need to access fields by name, you should create a single dictionary to maintain the relationship between names and column numbers.
public class SelectList<TSource>
{
private List<LambdaExpression> members = new List<LambdaExpression>();
public SelectList<TSource> Add<TValue>(Expression<Func<TSource, TValue>> selector)
{
members.Add(selector);
return this;
}
public Expression<Func<TSource, TResult>> ToDynamicColumns()
{
var parameter = Expression.Parameter(typeof(TSource), "e");
return Expression.Lambda<Func<TSource, object[]>>(
Expression.NewArrayInit(
typeof(object),
members.Select(m =>
Expression.Convert(Expression.Invoke(m, parameter), typeof(object))
)
),
parameter);
}
}
Though in your case, since you are writing an extension method to only return the same key details and a single additional field, you could probably define a single generic type to hold the results, and avoid any mucking around with Linq expressions at all;
public class UserResult<V>{
public int Id { get; set; }
public string DisplayName { get; set; }
public V Value { get; set; }
}
public static IQueryable<UserResult<V>> SelectCustom<V>(this IQueryable<User> query, Expression<Func<User, V>> ValueGetter)
{
return query.Select(u => new UserResult<V>{
Id = u.Id,
DisplayName = u.DisplayName,
Value = ValueGetter(u)
});
}
Well almost, if c# would just allow you to compile a call of one Expression<Delegate> from within another. Instead we can implement an ExpressionVisitor to unwrap any call to Compile;
public class DontCompile : ExpressionVisitor
{
protected override Expression VisitMember(MemberExpression node)
{
// Inline any lambda arguments that are expressions
if (node.Expression is ConstantExpression lambdaArgs
&& node.Member is FieldInfo field
&& typeof(Expression).IsAssignableFrom(field.FieldType))
return (Expression)field.GetValue(lambdaArgs.Value);
return base.VisitMember(node);
}
protected override Expression VisitMethodCall(MethodCallExpression node)
{
// Don't compile lambda expressions
if (node.Method.Name == "Compile"
&& typeof(LambdaExpression).IsAssignableFrom(node.Object.Type))
return Visit(node.Object);
return base.VisitMethodCall(node);
}
public static Expression<T> Tidy<T>(Expression<T> func) => (Expression<T>)new DontCompile().Visit(func);
}
...
return query.Select(DontCompile.Tidy<...>(u => new UserResult<V>{
Id = u.Id,
DisplayName = u.DisplayName,
Value = ValueGetter.Compile()(u)
});
...
But that's starting to look a bit messy.
You could do this with Expression.ListInit, here the TResult must have an Add instance method e.g. Dictionary<string, object>. This could just work but I havent even compiled it. In any way this should give enough hint on how to build this the way you want.
public Expression<Func<TSource, Dictionary<string, object>>> ToDynamicColumns()
{
var addMethod = typeof(TResult).GetMethod("Add");
var paramX = Expression.Parameter(typeof(TSource), "e");
var bindings =
this.members.Select (
member =>
return Expression.ElementInit(
addMethod,
Expression.Constant(mem.Name),
Expression.Convert(Expression.Property(paramX, member), typeof(object))
);
)
var listInit = Expression.ListInit(
Expression.New(typeof(TResult)),
bindings
);
return Expression.Lambda<Func<TSource, Dictionary<string, object>>(
listInit,
paramX
);
}
I'm trying to implement a generic item filter using Entity Framework based on a lambda expression telling me the id field. The following code compiles but of course does not work since EF does not understand the generic function:
public static IQueryable<T> Authorize<T>(this IQueryable<T> items, Func<T, Guid> idGetter) where T : class
{
return items.Where(i => idGetter(i) == new Guid("4A6FE5AF-AB63-4BB3-9D32-88766CF242CC"));
}
var result = context.Items.Authorize(i => i.Id);
How to do this using EF? How to use the expression tree to tell him what field to compare? How to use a generic name in a query that can be handled by Entity Framework?
If your id field is always of type Guid (as your code is suggesting), your classes could implement an interface with a member call Id of type Guid:
public interface MyInterface
{
Guid Id { get; set; }
}
public class Item : MyInterface
{
public Guid Id { get; set; }
// ...
}
Then your Authorize method could look like this:
public static IQueryable<T> Authorize<T>(this IQueryable<T> items) where T : MyInterface
{
return items.Where(i => i.Id == new Guid("4A6FE5AF-AB63-4BB3-9D32-88766CF242CC"));
}
And you can use this as follows:
var result = items.AsQueryable().Authorize();
Based on your comments and using reflection this is my approach:
First, your (simple) Attribute to decorate your "id fields":
[AttributeUsage(AttributeTargets.Property)]
public class IdFieldAttribute : Attribute
{
public IdFieldAttribute()
{
}
}
Then your classes could look like this:
public class Item
{
[IdFieldAttribute]
public Guid YourIdField{ get; set; }
// ...
}
A Reflection helper class for getting your desired field:
public static class ReflectionHelper
{
public static PropertyInfo GetPropertyInfoByAttribute<T>(T o, Type attributeType) where T : class
{
var type = o.GetType();
var propertyInfo = type.GetProperties(BindingFlags.Public | BindingFlags.Instance)
.FirstOrDefault(pi => Attribute.IsDefined(pi, attributeType));
return propertyInfo;
}
}
Once you have this, your Authorize method could look like this:
public static IQueryable<T> Authorize<T>(this IQueryable<T> items) where T : class
{
return items.Where(i => (Guid)ReflectionHelper.GetPropertyInfoByAttribute(i, typeof(IdFieldAttribute)).GetValue(i) == new Guid("4A6FE5AF-AB63-4BB3-9D32-88766CF242CC"));
}
And you can use it as follows:
var result = items.AsQueryable().Authorize();
I don't know if what you're looking for is something like this. Maybe this approach could help you.
I was thinking to generate EntityTypeConfiguration dynamically from run time and i don't want any EF dependency in Models[That is why i avoid Data Annotation].
So I declare a custom attribute(or can be any configuration file later on)
[AttributeUsage(AttributeTargets.Property, AllowMultiple=true )]
public class PersistableMemberAttribute : Attribute
{
public bool Iskey;
public bool IsRequired;
public bool IsIgnored;
public bool IsMany;
public string HasForeignKey;
public bool PropertyIsRequired;
public bool PropertyIsOptional;
}
And here is one of my Models is look like:
public class Blog
{
[PersistableMember(Iskey=true)]
public Guid BlogId { get; set; }
[PersistableMember(PropertyIsRequired = true)]
public string Name { get; set; }
public string Url { get; set; }
[PersistableMember(IsIgnored=true)]
public int Rating { get; set; }
[PersistableMember(IsMany =true)]
public ICollection<Post> Posts { get; set; }
}
Now I am going to write a generic EntityTypeConfiguration , which will create the configuration dynamically on run time based on the attribute values :
public class GenericEntityConfiguration<T> : EntityTypeConfiguration<T> where T : class
{
public GenericEntityConfiguration()
{
var members = typeof(T).GetProperties();
if (null != members)
{
foreach (var property in members)
{
var attrb= property.GetCustomAttributes(typeof( PersistableMemberAttribute ),false).OfType<PersistableMemberAttribute>();
if (attrb != null && attrb.Count() > 0)
{
foreach (var memberAttributute in attrb)
{
if (memberAttributute.Iskey || memberAttributute.IsIgnored)
{
var entityMethod = this.GetType().GetMethod("Setkey");
entityMethod.MakeGenericMethod(property.PropertyType)
.Invoke(this, new object[] { property, memberAttributute });
}
if (memberAttributute.IsRequired)
{
var entityMethod = this.GetType().GetMethod("SetRequired");
entityMethod.MakeGenericMethod(property.PropertyType)
.Invoke(this, new object[] { property, memberAttributute });
}
if (memberAttributute.PropertyIsRequired || memberAttributute.PropertyIsOptional)
{
var entityMethod = this.GetType().GetMethod("SetPropertyConfiguration");
entityMethod.MakeGenericMethod(property.PropertyType)
.Invoke(this, new object[] { property, memberAttributute });
}
}
}
}
}
}
public void SetPropertyConfiguration<TResult>(PropertyInfo propertyInfo, PersistableMemberAttribute attribute)
{
var functorParam = Expression.Parameter(typeof(T));
var lambda = Expression.Lambda(
Expression.Property(functorParam, propertyInfo)
, functorParam);
if (attribute.PropertyIsRequired)
{
this.Property<TResult>((Expression<Func<T, TResult>>)lambda).IsRequired();
}
if (attribute.PropertyIsOptional)
{
this.Property<TResult>((Expression<Func<T, TResult>>)lambda).IsOptional();
}
}
public void Setkey<TResult>(PropertyInfo propertyInfo, PersistableMemberAttribute attribute)
{
var functorParam = Expression.Parameter(typeof(T));
var lambda = Expression.Lambda(
Expression.Property(functorParam, propertyInfo)
, functorParam);
if (attribute.Iskey)
{
this.HasKey<TResult>((Expression<Func<T,TResult>>)lambda);
}
if (attribute.IsIgnored)
{
this.Ignore<TResult>((Expression<Func<T, TResult>>)lambda);
}
}
public void SetRequired<TResult>(PropertyInfo propertyInfo, PersistableMemberAttribute attribute) where TResult : class
{
var functorParam = Expression.Parameter(typeof(T));
var lambda = Expression.Lambda(
Expression.Property(functorParam, propertyInfo)
, functorParam);
if (attribute.IsRequired)
{
this.HasRequired<TResult>((Expression<Func<T, TResult>>)lambda);
}
}
}
But i got the compilation error of
Error 1 The type 'TResult' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'System.Data.Entity.ModelConfiguration.Configuration.StructuralTypeConfiguration.Property(System.Linq.Expressions.Expression>)' D:\R&D\UpdateStorePOC\UpdateStorePOC\Data\GenericEntityConfiguration.cs 63 17 UpdateStorePOC
which for these two statements:
this.Property<TResult>((Expression<Func<T, TResult>>)lambda).IsRequired();
this.Property<TResult>((Expression<Func<T, TResult>>)lambda).IsOptional();
that means that I need to put a constraint on my method to restrict it to a value type. In C#, this is done with the ‘struct’ keyword.
public void SetPropertyConfiguration<TResult>(PropertyInfo propertyInfo, PersistableMemberAttribute attribute) Where TResult : struct
But Its not the solution since my property type can be a class e.g string or int, bool double, etc . So it is not at all clear that I can send them into this method. Please help me to solve this issue whether there is any other way to do it.
I don't want any EF dependency in models.
With fluent mapping you're almost there and you won't come any closer. Your attributes, even though intended to be moved to a configuration file, don't make your model any more free of any EF footprint.1 Worse, they only add a second mapping layer (if you like) between your model and EF's mapping. I only see drawbacks:
You still have to maintain meta data for your model, probably not any less than regular fluent mapping and (probably) in awkward manually edited XML without compile-time checking.
You will keep expanding your code to cover cases that EF's mapping covers but yours doesn't yet.2 So it's a waste of energy: in the end you'll basically have rewritten EF's mapping methods.
You'll have to keep your fingers crossed when you want to upgrade EF.
With bugs/problems you're on your own: hard to get support from the community.
So my answer to your question help me to solve this issue would be: use fluent mapping out of the box. Keep it simple.
1 For example, you would still have to use the virtual modifier to enable proxies for lazy loading.
2 Like support for inheritance, unmapped foreign keys, max length, db data type, ... this could go on for a while.
I'd like to define in class declaration which items are index, something like:
public class MyClass {
public int SomeNum { get; set; }
[THISISANINDEX]
public string SomeProperty { get; set; }
}
so to have the same effect as ensureIndex("SomeProperty")
Is this possible?
I think this is a nice idea, but you have to do this yourself, there's no built-in support for it. If you have an access layer you can do it in there. You'd need an attribute class, something like this;
public enum IndexConstraints
{
Normal = 0x00000001, // Ascending, non-indexed
Descending = 0x00000010,
Unique = 0x00000100,
Sparse = 0x00001000, // allows nulls in the indexed fields
}
// Applied to a member
[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field)]
public class EnsureIndexAttribute : EnsureIndexes
{
public EnsureIndex(IndexConstraints ic = IndexConstraints.Normal) : base(ic) { }
}
// Applied to a class
[AttributeUsage(AttributeTargets.Class)]
public class EnsureIndexesAttribute : Attribute
{
public bool Descending { get; private set; }
public bool Unique { get; private set; }
public bool Sparse { get; private set; }
public string[] Keys { get; private set; }
public EnsureIndexes(params string[] keys) : this(IndexConstraints.Normal, keys) {}
public EnsureIndexes(IndexConstraints ic, params string[] keys)
{
this.Descending = ((ic & IndexConstraints.Descending) != 0);
this.Unique = ((ic & IndexConstraints.Unique) != 0); ;
this.Sparse = ((ic & IndexConstraints.Sparse) != 0); ;
this.Keys = keys;
}
}//class EnsureIndexes
You could then apply attributes at either the class or member level as follows. I found that adding at member level was less likely to get out of sync with the schema compared to adding at the class level. You need to make sure of course that you get the actual element name as opposed to the C# member name;
[CollectionName("People")]
//[EnsureIndexes("k")]// doing it here would allow for multi-key configs
public class Person
{
[BsonElement("k")] // name mapping in the DB schema
[BsonIgnoreIfNull]
[EnsureIndex(IndexConstraints.Unique|IndexConstraints.Sparse)] // name is implicit here
public string userId{ get; protected set; }
// other properties go here
}
and then in your DB access implementation (or repository), you need something like this;
private void AssureIndexesNotInlinable()
{
// We can only index a collection if there's at least one element, otherwise it does nothing
if (this.collection.Count() > 0)
{
// Check for EnsureIndex Attribute
var theClass = typeof(T);
// Walk the members of the class to see if there are any directly attached index directives
foreach (var m in theClass.GetProperties(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.FlattenHierarchy))
{
List<string> elementNameOverride = new List<string>(1);
EnsureIndexes indexAttr = null;
// For each members attribs
foreach (Attribute attr in m.GetCustomAttributes())
{
if (attr.GetType() == typeof(EnsureIndex))
indexAttr = (EnsureIndex)attr;
if (attr.GetType() == typeof(RepoElementAttribute))
elementNameOverride.Add(((RepoElementAttribute)attr).ElementName);
if ((indexAttr != null) && (elementNameOverride.Count != 0))
break;
}
// Index
if (indexAttr != null)
{
if (elementNameOverride.Count() > 0)
EnsureIndexesAsDeclared(indexAttr, elementNameOverride);
else
EnsureIndexesAsDeclared(indexAttr);
}
}
// Walk the atributes on the class itself. WARNING: We don't validate the member names here, we just create the indexes
// so if you create a unique index and don't have a field to match you'll get an exception as you try to add the second
// item with a null value on that key
foreach (Attribute attr in theClass.GetCustomAttributes(true))
{
if (attr.GetType() == typeof(EnsureIndexes))
EnsureIndexesAsDeclared((EnsureIndexes)attr);
}//foreach
}//if this.collection.count
}//AssureIndexesNotInlinable()
EnsureIndexes then looks like this;
private void EnsureIndexesAsDeclared(EnsureIndexes attr, List<string> indexFields = null)
{
var eia = attr as EnsureIndexes;
if (indexFields == null)
indexFields = eia.Keys.ToList();
// use driver specific methods to actually create this index on the collection
var db = GetRepositoryManager(); // if you have a repository or some other method of your own
db.EnsureIndexes(indexFields, attr.Descending, attr.Unique, attr.Sparse);
}//EnsureIndexes()
Note that you'll place this after each and every update because if you forget somewhere your indexes may not get created. It's important to ensure therefore that you optimise the call so that it returns quickly if there's no indexing to do before going through all that reflection code. Ideally, you'd do this just once, or at the very least, once per application startup. So one way would be to use a static flag to track whether you've already done so, and you'd need additional lock protection around that, but over-simplistically, it looks something like this;
void AssureIndexes()
{
if (_requiresIndexing)
AssureIndexesInit();
}
So that's the method you'll want in each and every DB update you make, which, if you're lucky would get inlined by the JIT optimizer as well.
See below for a naive implementation which could do with some brains to take the indexing advice from the MongoDb documentation into consideration. Creating indexes based on queries used within the application instead of adding custom attributes to properties might be another option.
using System;
using System.Reflection;
using MongoDB.Bson.Serialization.Attributes;
using MongoDB.Driver;
using NUnit.Framework;
using SharpTestsEx;
namespace Mongeek
{
[TestFixture]
class TestDecorateToEnsureIndex
{
[Test]
public void ShouldIndexPropertyWithEnsureIndexAttribute()
{
var server = MongoServer.Create("mongodb://localhost");
var db = server.GetDatabase("IndexTest");
var boatCollection = db.GetCollection<Boat>("Boats");
boatCollection.DropAllIndexes();
var indexer = new Indexer();
indexer.EnsureThat(boatCollection).HasIndexesNeededBy<Boat>();
boatCollection.IndexExists(new[] { "Name" }).Should().Be.True();
}
}
internal class Indexer
{
private MongoCollection _mongoCollection;
public Indexer EnsureThat(MongoCollection mongoCollection)
{
_mongoCollection = mongoCollection;
return this;
}
public Indexer HasIndexesNeededBy<T>()
{
Type t = typeof (T);
foreach(PropertyInfo prop in t.GetProperties() )
{
if (Attribute.IsDefined(prop, typeof (EnsureIndexAttribute)))
{
_mongoCollection.EnsureIndex(new[] {prop.Name});
}
}
return this;
}
}
internal class Boat
{
public Boat(Guid id)
{
Id = id;
}
[BsonId]
public Guid Id { get; private set; }
public int Length { get; set; }
[EnsureIndex]
public string Name { get; set; }
}
internal class EnsureIndexAttribute : Attribute
{
}
}