How to: Select Text in the Windows Forms TextBox Control
You can select text programmatically in the Windows Forms TextBox control. For example, if you create a function that searches text for a particular string, you can select the text to visually alert the reader of the found string's position.
To select text programmatically
Set the SelectionStart property to the beginning of the text you want to select.
The SelectionStart property is a number that indicates the insertion point within the string of text, with 0 being the left-most position. If the SelectionStart property is set to a value equal to or greater than the number of characters in the text box, the insertion point is placed after the last character.
Set the SelectionLength property to the length of the text you want to select.
The SelectionLength property is a numeric value that sets the width of the insertion point. Setting the SelectionLength to a number greater than 0 causes that number of characters to be selected, starting from the current insertion point.
(Optional) Access the selected text through the SelectedText property.
The code below selects the contents of a text box when the control's Enter event occurs. This example checks if the text box has a value for the Text property that is not null or an empty string. When the text box receives the focus, the current text in the text box is selected. The TextBox1_Enter event handler must be bound to the control; for more information, see How to: Create Event Handlers at Run Time for Windows Forms.
To test this example, press the Tab key until the text box has the focus. If you click in the text box, the text is unselected.
TextBox Control Overview (Windows Forms)
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TextBox Control (Windows Forms)