cannot convert from 'System.Linq.Expressions.Expression<System.Func<T, V?>>' to 'System.Linq.Expressions.Expression<System.Func<T, decimal>>'

Question

Why doesn't the following compile?

using System;
using System.Linq;
using System.Linq.Expressions;

public static class Extensions
{
    public static V? SumOrDefault<T, V>(this IQueryable<T> @this, Expression<Func<T, V>> selector)
        where V : struct, IComparable, IComparable<V>, IConvertible, IEquatable<V>, IFormattable
    {
        Expression<Func<T, V?>> nullableSelector = null; // omitted for brevity
        return Queryable.Sum<T>(@this, nullableSelector);
    }
}

It gives this error:

error CS1503: Argument 2: cannot convert from 'System.Linq.Expressions.Expression<System.Func<T, V?>>' to 'System.Linq.Expressions.Expression<System.Func<T, decimal>>'

Two questions:

• Why is it trying & failing to call the decimal version of Sum<> ?
• Why does it fail to find the decimal? version of that function in System.Linq.Queryable ?

        public static decimal? Sum<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, decimal?>> selector);

Answer 1

If that worked, technically, you could call that SumOrDefault function with something that decimal cannot convert yet still respect your where ( string a custom "unsummable" class would, for example).

And it can't find the decimal? version of the function because it doesn't know which type to convert to safely.

C# doesn't have a way to filter a type to just numerics, AFAIK, so you will have to overload them manually like it's 2001:

public static decimal? SumOrDefault<T>(this IQueryable<T> @this, Expression<Func<T, decimal>> selector)
public static double? SumOrDefault<T>(this IQueryable<T> @this, Expression<Func<T, double>> selector)

//etc.