Moq extension inspired by https://gist.github.com/7Pass/1c6b329e85ca29071f42. Allows mocks to be setup with all args as default without having to type out each one in the Setup.

_MoqExtensions.cs

public static class MoqExtensions
{
	public static ISetup<T, TResult> SetupDefaultArgs<T, TResult>(this Mock<T> mock, string methodName)
		where T : class
	{
		var method = typeof(T).GetMethod(methodName);
		if(method == null)
		{
			throw new ArgumentException($"No method named '{methodName}' exists on type '{typeof(T).Name}'");
		}
		var instance = Expression.Parameter(typeof(T), "m");
		var callExp = Expression.Call(instance, method, method.GetParameters().Select(p => GenerateItIsAny(p.ParameterType)));
		var exp = Expression.Lambda<Func<T, TResult>>(callExp, instance);
		return mock.Setup(exp);
	}

	private static MethodCallExpression GenerateItIsAny(Type T)
	{
		var ItIsAnyT = typeof(It)
			.GetMethod("IsAny")
			.MakeGenericMethod(T);
		return Expression.Call(ItIsAnyT);
	}
}

exampleUsage.cs

void Main()
{
	var mock = new Mock<IFooContainer>();
	mock.SetupDefaultArgs<IFooContainer, string>("DoBar")
		.Returns<string>((s) => s);

	var result = mock.Object.DoBar("it worked");
	Console.WriteLine(result);

	mock.SetupDefaultArgs<IFooContainer, string>("DoFoo")
		.Returns<string, int, Guid>((s, i, g) => $"{s} {i} {g}");

	result = mock.Object.DoFoo("it worked", 3, Guid.NewGuid());
	Console.WriteLine(result);

	result = mock.Object.DoFoo("it worked", 3, Guid.NewGuid());
	Console.WriteLine(result);

	// throws "No method named 'Invalid Method' exists on type 'IFooContainer'"
	mock.SetupDefaultArgs<IFooContainer, string>("Invalid Method")
		.Returns<string, int, Guid>((s, i, g) => $"{s} {i} {g}");
}

public interface IFooContainer
{
	string DoFoo(string a, int b, Guid id);
	string DoBar(string a);
}

Are there certain references that are needed for this? I get an The non-generic type 'ISetup' cannot be used with type arguments compiler error from the method SetupDefaultArgs.

Nevermind, I found it. It needs a reference to using Moq.Language.Flow;