In WealthLab Pro 5.3, Code Completion (Intellisense) for the class C and Method M in the code below works as expected. A list is displayed by the editor when the "." in "c.M" is typed and the method definition is displayed when the "(" in "c.M (this);" is typed.

CODE:
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When the code for class C is moved to a Visual C# 2008 Express project (into namespace X.Y), compiled, the DLL copied to the Wealth-Lab Pro 5 directory, a reference added to that assembly in WealthLab, "using X.Y" added to the WealthLab editor code and method M invoked by the Execute method of the recompiled code in the WealthLab editor, it runs as expected. The letter "M" is output to the Debug Log. However, Code Completion in the editor no longer appears to function as before. Is there something that can be done to correct this?

This is a known usability issue, unfortunately it had low priority to be fixed in 5.4. You can create a support ticket for tracking purposes.