Entity Framework - dynamic query - entity-framework

Question

I'm creating a dynamic expression builder and trying to implement the 'like' function. Before writing my own I've searched for any existing function and found one close to my need. After several experiments I couldn't bring it to run for types other than string.
When I pass a parameter of type int then I get this error:
Method 'System.String ToString()' declared on type 'System.String' cannot be called with instance of type 'System.Int32'
My code looks like this:
private static MethodCallExpression GetLowerCasePropertyAccess(MemberExpression propertyAccess)
{
//return Expression.Call(Expression.Call(propertyAccess, "ToString", new Type[0]), typeof(string).GetMethod("ToLower", new Type[0]));
return Expression.Call(Expression.Call(propertyAccess, typeof(string).GetMethod("ToString", System.Type.EmptyTypes)), typeof(string).GetMethod("ToLower", System.Type.EmptyTypes));
}
private static readonly MethodInfo ContainsMethod = typeof(String).GetMethod("Contains", new Type[] { typeof(String) });
public static Expression<Func<T, bool>> Create<T>(string propertyName, ComparisonOperators comparisonOperator, dynamic comparedValue1, dynamic comparedValue2 = null)
{
ParameterExpression parameterExpression = Expression.Parameter(typeof(T), "x");
MemberExpression memberExpression = Expression.MakeMemberAccess(parameterExpression, typeof(T).GetProperty(propertyName));
ConstantExpression constantExpression = Expression.Constant(comparedValue1, comparedValue1.GetType());
Expression expressionBody = null;
switch (comparisonOperator)
{
...
case ComparisonOperators.Contains:
//var indexOf = Expression.Call(memberExpression, "IndexOf", null, Expression.Constant(comparedValue1, typeof(string)), Expression.Constant(StringComparison.InvariantCultureIgnoreCase));
//expressionBody = Expression.GreaterThanOrEqual(indexOf, Expression.Constant(0));
expressionBody = Expression.Call(GetLowerCasePropertyAccess(memberExpression), ContainsMethod, Expression.Constant(comparedValue1.ToLower()));
break;
}
return Expression.Lambda<Func<T, bool>>(expressionBody, new ParameterExpression[] { parameterExpression });
}

Answer 1

I'm not sure I fully understand what you're doing, but I think your error is caused by this line:
return Expression.Call(Expression.Call(propertyAccess, typeof(string).GetMethod("ToString", System.Type.EmptyTypes)), typeof(string).GetMethod("ToLower", System.Type.EmptyTypes));
Which will always try and call the ToString method for a string type, so if you try and use an Int32 property, you're then trying to call String.ToString(), since the implementation of ToString() will be different for different types and the two implementations will not necessarily be compatible, you'll get the exception you're seeing:
Method 'System.String ToString()' declared on type 'System.String' cannot be called with instance of type 'System.Int32'
From what it looks like you're doing, I think this may be what you're after:
return Expression.Call(Expression.Call(propertyAccess, propertyAccess.Type.GetMethod("ToString", System.Type.EmptyTypes)), typeof(string).GetMethod("ToLower", System.Type.EmptyTypes));
Which will use the correct implementation of ToString (with type obtained from propertyAccess.Type).

Answer 2

Linq to entities doesn't support .ToString method. For converting numeric values to string you need to use SqlFunctions.StringConvert method. I've fixed your code and now you can do like on string and numeric columns:
private static Expression GetConvertToStringExpression(Expression e)
{
// if property string - no cast needed
// else - use SqlFunction.StringConvert(double?) or SqlFunction.StringConvert(decimal?);
Expression strExpression = null;
if (e.Type == typeof(string))
strExpression = e;
var systemType = Nullable.GetUnderlyingType(e.Type) ?? e.Type;
if (systemType == typeof(int)
|| systemType == typeof(long)
|| systemType == typeof(double)
|| systemType == typeof(short)
|| systemType == typeof(byte)) // continue
{
// cast int to double
var doubleExpr = Expression.Convert(e, typeof (double?));
strExpression = Expression.Call(StringConvertMethodDouble, doubleExpr);
}
if (systemType == typeof (decimal))
{
// call decimal version of StringConvert method
// cast to nullable decimal
var decimalExpr = Expression.Convert(e, typeof (decimal?));
strExpression = Expression.Call(StringConvertMethodDecimal, decimalExpr);
}
return strExpression;
}
private static MethodCallExpression GetLowerCasePropertyAccess(Expression propertyAccess)
{
var stringExpression = GetConvertToStringExpression(propertyAccess);
if (stringExpression == null)
throw new Exception(string.Format("Not supported property type {0}", propertyAccess.Type));
return Expression.Call(stringExpression,
typeof (string).GetMethod("ToLower", Type.EmptyTypes));
}
private static readonly MethodInfo StringConvertMethodDouble = typeof (SqlFunctions).GetMethod("StringConvert",
new Type[] {typeof (double?)});
private static readonly MethodInfo StringConvertMethodDecimal = typeof(SqlFunctions).GetMethod("StringConvert",
new Type[] { typeof(decimal?) });

Answer 3

I have just made something like that:
public Expression<Func<T,bool>> BuildContainsExpression<T>(MemberExpression memberExp, object comparedValue)
{
var parameter = Expression.Parameter(memberExp.Member.DeclaringType, "x");
var method = typeof(string).GetMethod("Contains", types: new[] { typeof(string) });
var comparison = Expression.Equal(
Expression.Call(
method: method,
instance: memberExp,
arguments: Expression.Constant(comparedValue)),
Expression.Constant(true)
);
return Expression.Lambda<Func<T, bool>>(comparison, parameter);
}
And it builds an expression like below:
x.Language.Contains("tr")
(with my dynamic parameters)