Develop Entity Framework class that will define a .mdf database - entity-framework

Question

I am trying to develop Entity Framework class that will define a .mdf database already created and filled leading up to actually creating one and filling it.
Error:
One or more validation errors were detected during model generation:
testclass.test: : EntityType 'test' has no key defined. Define the key for this EntityType.
testData: EntityType: EntitySet 'testData' is based on type 'test' that has no keys defined.
Code:
using System.Data.Entity;
namespace testclass
{
public class test
{
public int idt { get; set; }
public string datetime { get; set; }
public string col1 { get; set; }
public string col2 { get; set; }
public string col3 { get; set; }
}
public class TestDbContext : DbContext
{
public DbSet<test> testData { get; set; }
}
public class testRepository
{
public List<test> Gettest()
{
TestDbContext testDbContext = new TestDbContext();
return testDbContext.testData.ToList();
}
}
}
connection string:
<connectionStrings>
<add name="TestDbContext" connectionString="Data Source=.\SQLEXPRESS;Integrated Security=SSPI;AttachDBFilename=C:\folders\testclass\testclass\App_Data\Data.mdf;User Instance=true;" providerName="System.Data.SqlClient" />
</connectionStrings>

Answer 1

There is some issues with your code:
the main is the lack of a primary key on your entity: EF can infer some keys if their names match some conventions : "Id", "ID", "testId"... So either use one of these standard names or explicitly mark your property as a primary key with the [Key] attribute:
[Key]
public int idt { get; set; }
to ensure proper cleanup of the resources allocated by the DbContext you should wrap its usage in a using block:
using (TestDbContext testDbContext = new TestDbContext())
{
return testDbContext.testData.ToList();
}
in .Net the naming conventions are: CamelCase for everything except local variables and fields; so testclass should be Testclass, col1 should be Col1, testData should be TestData...