How to get the location of the DLL currently executing?

You are looking for System.Reflection.Assembly.GetExecutingAssembly()

string assemblyFolder = Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location);
string xmlFileName = Path.Combine(assemblyFolder,"AggregatorItems.xml");

Note:

The .Location property returns the location of the currently running DLL file.

Under some conditions the DLL is shadow copied before execution, and the .Location property will return the path of the copy. If you want the path of the original DLL, use the Assembly.GetExecutingAssembly().CodeBase property instead.

.CodeBase contains a prefix (file:\), which you may need to remove.

Reflection is your friend, as has been pointed out. But you need to use the correct method;

Assembly.GetEntryAssembly()     //gives you the entrypoint assembly for the process.
Assembly.GetCallingAssembly()   // gives you the assembly from which the current method was called.
Assembly.GetExecutingAssembly() // gives you the assembly in which the currently executing code is defined
Assembly.GetAssembly( Type t )  // gives you the assembly in which the specified type is defined.