Please try the following code line before
you add the data source:
| ((RadGridView)this.radMultiColumnComboBox1.EditorControl).MasterGridViewTemplate.AutoGenerateColumns = false;
Since you are adding the data source at run time, the above should work, but if this does not help, please open a new support ticket and send us a sample project which we can test locally.
All the best,
the Telerik team
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