exploring C# 8

switch.cs

public static IArgumentMatcher? Create(Expression arg) => arg switch
    {
        MethodCallExpression call when call.Method.DeclaringType == typeof(Arg) => call.Method.Name switch
        {
            nameof(Arg.Ignore) => (IArgumentMatcher)new IgnoreArgument(),
            nameof(Arg.Where) => new LambdaArgument(call.Arguments.Single()),
            _ => throw new InvalidOperationException()
        },
        _ => null
    };

switch_before.cs

public static IArgumentMatcher Create(Expression arg)
{
    if (arg is MethodCallExpression call && call.Method.DeclaringType == typeof(Arg))
    {
        switch (call.Method.Name)
        {
            case "Ignore":
                return new IgnoreArgument();
            case "Equals":
                return new EqualsArgument(call.Arguments[0]);
            default:
                throw new NotImplementedException();
        }
    }

    return null;
}