XmlSerializer doesn't work with Dictionaries. Oh, and it has problems with KeyValuePairs too.

Why not use the DataContractSerializer instead?

Dictionary<string, string> dict = new Dictionary<string,string>();
dict.Add("myKey", "myValue");

MemoryStream memoryStream = new MemoryStream();
DataContractSerializer serializer = new DataContractSerializer(dict.GetType());
serializer.WriteObject(memoryStream, dict);

memoryStream.Position = 0;
StreamReader reader = new StreamReader(memoryStream);
string str = reader.ReadToEnd();

str will look like

<ArrayOfKeyValueOfstringstring xmlns="http://schemas.microsoft.com/2003/10/Serialization/Arrays" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
<KeyValueOfstringstring>
<Key>myKey</Key>
<Value>myValue</Value>
</KeyValueOfstringstring>
</ArrayOfKeyValueOfstringstring>