Looking for some code samples how to pass DDE messages from a .NET program I found out that there‘s a great library already in place that facilitates working with DDE: the NDde project from Codeplex: http://ndde.codeplex.com/
However, I just needed to make one call and including a new DLL into my code would have been too much.
So I just created a sample C# program derived from the NDde code that imports only the necessary functions from the Win32 API to pass an Open DDE message to Excel for opening a workbook.
Workbook wb = DDEHelper.OpenWorkbook(_excel, args);
D:\Projects\DDE>csc /r:C:\Windows\assembly\GAC_MSIL\Microsoft.Office.Interop.Excel\188.8.131.52__71e9bce111e9429c\Microsoft.Office.Interop.Excel.dll DDEOpen.cs