Filtering operation in RadMultiColumnComboBox is controlled by the boolean AutoFilter property. The data is filtered by the field specified as DisplayMember. In addtion you should set the filter expression (telling the control the rules it should use to filter the data) and add that expression to MasterTemplate:
Copy[C#]
this.radMultiColumnComboBox1.AutoFilter = true;
this.radMultiColumnComboBox1.DisplayMember = "ContactName";
FilterDescriptor filter = new FilterDescriptor();
filter.PropertyName = this.radMultiColumnComboBox1.DisplayMember;
filter.Operator = FilterOperator.Contains;
filter.Value = "Maria";
this.radMultiColumnComboBox1.EditorControl.MasterTemplate.FilterDescriptors.Add(filter);
Copy[VB.NET]
Me.RadMultiColumnComboBox1.AutoFilter = True
Me.RadMultiColumnComboBox1.DisplayMember = "ContactName"
Dim filter As New FilterDescriptor()
filter.PropertyName = Me.RadMultiColumnComboBox1.DisplayMember
filter.Operator = FilterOperator.Contains
filter.Value = "Maria"
Me.RadMultiColumnComboBox1.EditorControl.MasterTemplate.FilterDescriptors.Add(filter)
The code above will result in the following multi-column combobox: