one generic class create generic extension with expression<T,bool> method

Question

My code is as follows:

public partial class WhereHelper<T1> { }
public static partial class WhereHelperExtension
{
    public static T Where<T,T1>(this T t, Expression<Func<T1,bool>> where) where T : WhereHelper<T1>
    {
        //do something....
        return t;
    }
}
public class Test
{
    public void Main()
    {
        WhereHelper<DateTime> dt = new WhereHelper<DateTime>();
        dt.Where(t => t.Year == 2016);//this is error
        dt.Where<WhereHelper<DateTime>, DateTime>(t => t.Year == 2016);//this is success
    }
}

Extension method I want to be like this：

WhereHelper<DateTime> dt = new WhereHelper<DateTime>();
dt.Where(t => t.Year == 2016);//this is error

how to create generic extension with Expression method. Visual Studio does not recognize the "Where" extension methods.

Answer 1

In C#, if you need to provide any generic argument, you must provide them all. where constraints do not provide hints to the type resolver, and so it's impossible to decide what T1 is.

Change your signature to the following:

public static WhereHelper<T> Where<T>(this WhereHelper<T> t, Expression<Func<T,bool>> where)
{
    return t;
}

Here, we know exactly what T , purely from the first argument, and so we do not have to explicitly specific the arguments.