TypeClass.cs

using System;
using System.Collections.Generic;
using System.Linq;

var coll = Enumerable.Range(1, 10);

// We need to repeat "int" here, it can't be implied by IntAdd or IntMul :(
Console.WriteLine($"The sum is {AppendMany<IntAdd, int>(coll)}");
Console.WriteLine($"The product is {AppendMany<IntMul, int>(coll)}");

TValue AppendMany<TMonoid, TValue>(IEnumerable<TValue> xs) where TMonoid : IMonoid<TValue>
{
    var acc = TMonoid.Empty;
    foreach (var x in xs)
    {
        acc = TMonoid.Append(acc, x);
    }
    return acc;
}

interface IMonoid<TValue>
{
    static abstract TValue Empty { get; }
    static abstract TValue Append(TValue x, TValue y);
}

class IntAdd : IMonoid<int>
{
    public static int Empty => 0;
    public static int Append(int x, int y) => x + y;
}

class IntMul : IMonoid<int>
{
    public static int Empty => 1;
    public static int Append(int x, int y) => x * y;
}