kb(dropdownlist):added kb for showing the default text only before an item is selected (#322)

Author: ntacheva

knowledge-base/dropdown-no-default-text-on-selection.md

@@ -0,0 +1,54 @@
+---
+title: Show Default Text only before an item is selected
+description: How to modify the Default Text to be shown only before an item is selected
+type: how-to
+page_title: Show Default Text only before an item is selected
+slug: dropdown-kb-no-default-text-on-selection
+position: 
+tags: telerik,blazor,dropdownlist,default,text
+ticketid: 1518898
+res_type: kb
+---
+
+## Environment
+<table>
+	<tbody>
+		<tr>
+			<td>Product</td>
+			<td>DropDownList for Blazor</td>
+		</tr>
+	</tbody>
+</table>
+
+
+## Description
+
+I want the DropDownList to show the default text only when no option is selected. E.g. once the user selects a value, they are not allowed to select 'null' again.
+
+## Solution
+
+You can modify the default text to be shown only before an item is selected by toggling the DefaultText parameter value depending on your business logic. The example below demonstrates how to achieve that.
+
+````CSHTML
+@* Show the default text only when no option is selected *@
+
+Selected value: @selectedValue
+<br />
+
+<TelerikDropDownList Data="@myDdlData" TextField="MyTextField" ValueField="MyValueField" @bind-Value="selectedValue"
+                     DefaultText="@( selectedValue == 0 ? "Please Select" : null )">
+</TelerikDropDownList>
+
+@code {
+
+    int selectedValue { get; set; }
+
+    IEnumerable<MyDdlModel> myDdlData = Enumerable.Range(1, 20).Select(x => new MyDdlModel { MyTextField = "item " + x, MyValueField = x });
+
+    public class MyDdlModel
+    {
+        public int MyValueField { get; set; }
+        public string MyTextField { get; set; }
+    }
+}
+````