Attached to https://help.citavi.com/topic/163066-citavi-kategorien-mittels-excel-import-makro-cim007-verandern

setCategoryCitaviMakro.cs

using System;
using System.Linq;
using System.ComponentModel;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Text.RegularExpressions;
using System.Net;

using SwissAcademic.Citavi;
using SwissAcademic.Citavi.Metadata;
using SwissAcademic.Citavi.Shell;
using SwissAcademic.Collections;
using SwissAcademic.Citavi.DataExchange;

// Implementation of macro editor is preliminary and experimental.
// The Citavi object model is subject to change in future version.

public static class CitaviMacro
{
	public static void Main()
	{
		
		Project project = Program.ActiveProjectShell.Project; //Get the active project
		MainForm mainForm = Program.ActiveProjectShell.PrimaryMainForm; //Get the active ("primary") MainForm
		ProjectReferenceCollection references = project.References; //if this macro should ALWAYS affect all titles in active project, choose:		 

		
		// Create List with all categories
		List<Category> allCategories = project.AllCategories.ToList();
		
		
		// Run through references		
		foreach (Reference reference in references)
		{
			DebugMacro.WriteLine(reference.ShortTitle);
			// DebugMacro.WriteLine(reference.Categories);
			
			string setCategoryFullString = "Kat1; Kat2";
			
			// Split setCategoryFullString into category list setCategoryList 
			List<string> setCategoryList = setCategoryFullString.Split(new String[]{"; "}, StringSplitOptions.RemoveEmptyEntries).ToList();
			setCategoryList.RemoveAll(string.IsNullOrWhiteSpace);
			
			
			foreach (string setCategoryString in setCategoryList)
			{
				// Add category for this reference
				List<Category> categoriesToAdd = new List<Category>();
				var addThisCategory = allCategories.Where(item => item.FullName.Equals(setCategoryString, StringComparison.InvariantCulture)).FirstOrDefault();
				categoriesToAdd.Add(addThisCategory);
				
				if(addThisCategory != null && !reference.Categories.Contains(addThisCategory))
				{
					reference.Categories.AddRange(categoriesToAdd);	   // Add categories to this reference
					DebugMacro.WriteLine("   Set to category " + addThisCategory);
				} 
				else if (reference.Categories.Contains(addThisCategory))
				{
					DebugMacro.WriteLine("   is already category " + addThisCategory);
				}
				
			}	
		}
	}
}