两个具有相同名称的扩展方法

Question

I have the following two extension methods (the body of the methods is not particularly important for my question, but including the code anyway)

public static class DictionaryExtensions
{
    public static TValue GetValue<TKey, TValue>(this IDictionary<TKey, TValue> source, TKey key, TValue defaultValue)
    {
        return (source.ContainsKey(key) ? source[key] : defaultValue);
    }
}

public static class WebExtensions
{
    public static T GetValue<T>(this HttpContext context, string name, T defaultValue)
    {
        object value = context.Request.Form[name] ?? context.Request.QueryString[name];
        if (value == null) return defaultValue;
        return (T)value;
    }
}

These two methods share the same name but they extend two very different types. I would expect the following code to be simple and the compiler to be able to pick the appropriate extension method:

var myDict = new Dictionary<int, string>()
{
    { 1, "foo" },
    { 2, "bar" }
};
var result = myDict.GetValue(5, "baz");

However, for some unknown reason, Visual Studio refuses to compile my code with the following compile-time error: "The type 'System.Web.HttpContext' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Web'" . This error tells me that the compiler selected the GetValue extension method in the WebExtensions class instead of the one in the DictionaryExtension class.

I was able to get around the problem with the following:

var result = DictionaryExtensions.GetValue(myDict, 5, "baz");

but I'm trying to understand why the compiler was confused in the first place. Does anybody know why?

Answer 1

Another option is to break your 2 Extension classes out into separate namespaces and do not add a using statement where the WebExtensions class resides in your consuming code. That way the compiler won't try and resolve GetValue to WebExtensions .

Answer 2

Just do what the compiler says and add a reference to System.Web.dll. Then it should compile.

Your external assembly where these extension method classes exist take a dependency on System.Web.dll. The compiler should be able to figure out the overload, but in order to do so, it needs to reference the assembly where HttpContext exists, which is in System.Web.dll.

On the other hand if you don't want your project that consumes the extension method assembly to take a dependency on System.Web.dll, then you need to define those 2 extension method classes in separate assemblies.

@Steve Danner has a good solution too : put the two extension method classes into different namespaces in the same assembly, and omit the using statement for the namespace that contains the WebExtensions class.

I run into this occasionally with things like System.Runtime.Serialization, System.ServiceModel.Activation, etc. It can happen when you have a dependency that is twice-removed. In other words your root project depends on an assembly which depends on a type another third assembly.