Can't return Dictionary(Of String, String) via GET ajax web request, works with POST

I'm a little confused - if the ResponseFormat is JSON (as in your example above) then IDictionary derivatives should be supported, however if it is XML then I could understand seeing this error as the XmlSerializer does not support this.

One option to send a dictionary type using the XmlSerializer is to implement logic to convert it to an array or List or ArrayList. Alternatively you could implement a custom serializer for the data and write your own XML, returning an XmlDocument from your method instead. This would allow you to format the data in any way you choose.

Maybe you could clarify if you are using JSON or XML?

Another alternative is to change the Return type to a String and then convert the Dictionary to JSON via JavaScriptSerializer.Serialize. This may not be exactly what you were intending with returning a Dictionary but it will be key=value pairs in the JSON Response.

<WebMethod()> _
<ScriptMethod(ResponseFormat:=ResponseFormat.Json, UseHttpGet:=True, XmlSerializeString:=True)> _
Public Function GetDictionary() As String

    Dim d As New Dictionary(Of String, String)
    d.Add("key1", "value1")
    d.Add("key2", "value2")
    Return New JavaScriptSerializer().Serialize(d)

End Function

And the resulting JSON:

{"key1":"value1","key2":"value2"}