Do not nest generic types in member signatures

Question

I have acquired an extension class which implements the following members with signatures that are violating the CA1006:DoNotNestGenericTypesInMemberSignatures rule.

The code to which the warning refers is included below.

How should I refactor the code to resolve the CA1006 warning?

Please keep in mind that I am not very familiar with expression trees, although I do have a pretty good grasp of anonymous methods, delegates, and lambdas.

Any help will be greatly appreciated.

    public static DataServiceQuery<TElement> Expand<TElement, TPropType>(this DataServiceQuery<TElement> source, Expression<Func<TElement, TPropType>> propertySelector) 
    {
        string includeString = BuildString(propertySelector);
        return source.Expand(includeString);
    }

    private static string BuildString(Expression propertySelector)
    {
        switch (propertySelector.NodeType)
        {
            case ExpressionType.Lambda:
                LambdaExpression lambdaExpression = (LambdaExpression)propertySelector;
                return BuildString(lambdaExpression.Body);

            case ExpressionType.Quote:
                UnaryExpression unaryExpression = (UnaryExpression)propertySelector;
                return BuildString(unaryExpression.Operand);

            case ExpressionType.MemberAccess:

                MemberExpression memberExpression = (MemberExpression)propertySelector;
                MemberInfo propertyInfo = memberExpression.Member;

                if (memberExpression.Expression is ParameterExpression)
                {
                    return propertyInfo.Name;
                }
                else
                {
                    // we've got a nested property (e.g. MyType.SomeProperty.SomeNestedProperty)
                    return BuildString(memberExpression.Expression) + "/" + propertyInfo.Name;
                }

            case ExpressionType.Call:
                MethodCallExpression methodCallExpression = (MethodCallExpression)propertySelector;
                if (IsSubInclude(methodCallExpression.Method)) // check that it's a SubInclude call
                {
                    // argument 0 is the expression to which the SubInclude is applied (this could be member access or another SubInclude)
                    // argument 1 is the expression to apply to get the included property
                    // Pass both to BuildString to get the full expression
                    return BuildString(methodCallExpression.Arguments[0]) + "/" +
                           BuildString(methodCallExpression.Arguments[1]);
                }
                // else drop out and throw
                break;
        }
        throw new InvalidOperationException("Expression must be a member expression or an SubInclude call: " + propertySelector.ToString());

    }

    private static readonly MethodInfo[] SubIncludeMethods;
    static MyExtensions()
    {
        Type type = typeof(MyExtensions);
        SubIncludeMethods = type.GetMethods().Where(mi => mi.Name == "SubExpand").ToArray();
    }

    private static bool IsSubInclude(MethodInfo methodInfo)
    {
        if (methodInfo.IsGenericMethod)
        {
            if (!methodInfo.IsGenericMethodDefinition)
            {
                methodInfo = methodInfo.GetGenericMethodDefinition();
            }
        }
        return SubIncludeMethods.Contains(methodInfo);
    }

    public static TPropType SubExpand<TSource, TPropType>(this Collection<TSource> source, Expression<Func<TSource, TPropType>> propertySelector)
        where TSource : class
        where TPropType : class
    {
        throw new InvalidOperationException("This method is only intended for use with DataServiceQueryExtensions.Expand to generate expressions trees"); // no actually using this - just want the expression! 
    }

    public static TPropType SubExpand<TSource, TPropType>(this TSource source, Expression<Func<TSource, TPropType>> propertySelector)
        where TSource : class
        where TPropType : class
    {
        throw new InvalidOperationException("This method is only intended for use with DataServiceQueryExtensions.Expand to generate expressions trees"); // no actually using this - just want the expression! 
    }

Answer 1

The warning is a general warning supposed to help you design a better and simpler public interface. In this case you get a warning about having a Expression<Func<TElement, TPropType>> parameter in your method. However, for this method there is no point in simplifying the type and instead you should suppress the warning using an attribute or completely remove the rule from your ruleset.

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A silly example where you probably should consider following the advice of the rule is a method like this:

public void F(Dictionary<String, List<Tuple<String, Int32>>> dictionary);