Cannot convert from Func<T,T,T> to Func<T,T,T>

问题

I'm quite confused by this error:

Cannot implicitly convert type 'System.Func<T,T,T> [c:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0\mscorlib.dll]' to 'System.Func<T,T,T> [c:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0\mscorlib.dll]' path\to\my\project\Operators.cs

The types are identical, why is it even trying to do a cast? Here's the code:

public static class Operators<T>
{
    private static Func<T,T,T> _add = null;

    public static T Add<T>(T a, T b)
    {
        if (_add == null) {
            var param1Expr = Expression.Parameter(typeof (T));
            var param2Expr = Expression.Parameter(typeof (T));
            var addExpr = Expression.Add(param1Expr, param2Expr);
            var expr = Expression.Lambda<Func<T, T, T>>(addExpr, param1Expr, param2Expr);
            _add = expr.Compile(); // <--- error occurs here
        }
        return _add.Invoke(a, b);
    }
}

回答1:

The problem is that your method is generic, introducing a new type parameter T. So the T outside the method isn't the same as the T inside the method.

Just change your method to not be generic:

public static T Add(T a, T b)

... and it should be fine.

To be clearer, your code is currently equivalent to this:

public static class Operators<TC>
{
    private static Func<TC, TC, TC> _add = null;

    public static TM Add<TM>(TM a, TM b)
    {
        if (_add == null) {
            var param1Expr = Expression.Parameter(typeof(TM));
            var param2Expr = Expression.Parameter(typeof(TM));
            var addExpr = Expression.Add(param1Expr, param2Expr);
            var expr = Expression.Lambda<Func<TM, TM, TM>>
                          (addExpr, param1Expr, param2Expr);
            _add = expr.Compile();
        }
        return _add.Invoke(a, b);
    }
}

Note how I've renamed the T introduced by the class to TC, and the T introduced by the method to TM. The error message now looks more reasonable:

Test.cs(19,20): error CS0029: Cannot implicitly convert type
        'System.Func<TM,TM,TM>' to 'System.Func<TC,TC,TC>'

回答2:

The T for your Operators<T> class and the T type parameter for Add are different types, so there's no guarantee that the types are compatible.

For example you could do:

Operators<string>.Add<int>(1, 2);

The compiler emits a warning to this effect:

Type parameter 'T' has the same name as the type parameter from outer type 'Operators'

来源：https://stackoverflow.com/questions/15055900/cannot-convert-from-funct-t-t-to-funct-t-t