Add custom file properties programmatically [duplicate]

I need to add custom file properties (see here) to thousands of files programmatically.
The WindowsAPICodePack can get/set existing file properties, but it seems it can not add custom properties?!?

Here the code that works based on Add new metadata properties to a file:

You must reference the DSOFile.dll which can be downloaded from Microsoft here:
Microsoft Developer Support OLE File Property Reader 2.1 Sample

using DSOFile; OleDocumentProperties myFile = new DSOFile.OleDocumentProperties(); myFile.Open(@"c:\temp\B30700.asm", false, DSOFile.dsoFileOpenOptions.dsoOptionDefault); bool property_exists; object prop_value; prop_value = "999"; //Then check if there's already a property like the one you want to create property_exists = false; foreach (DSOFile.CustomProperty property in myFile.CustomProperties) { if (property.Name == "Your Property Name") { //Property exists //End the task here (return;) oder edit the property property_exists = true; property.set_Value(prop_value); } } if (!property_exists) myFile.CustomProperties.Add("Your Property Name", ref prop_value); myFile.Save(); myFile.Close(true);