To search for text in a RichTextBox control in C#, you can use the Find method. Here’s an example of how you can search for text in a RichTextBox:

private void SearchText(string searchText)
    int startPos = richTextBox1.SelectionStart + richTextBox1.SelectionLength;

    int index = richTextBox1.Find(searchText, startPos, RichTextBoxFinds.None);

    if (index != -1)
        richTextBox1.SelectionStart = index;
        richTextBox1.SelectionLength = searchText.Length;
        // Text not found
        MessageBox.Show("Text not found.");

In this example, the SearchText method takes a searchText parameter, which is the text you want to search for. It starts the search from the current selection position in the RichTextBox.

The Find method is called on the richTextBox1 control and searches for the specified text starting from the startPos. The RichTextBoxFinds.None option is used to perform a simple search without any special options.

If the text is found, the method sets the selection in the RichTextBox to highlight the found text and gives focus to the control. If the text is not found, it displays a message box indicating that the text was not found.

You can call the SearchText method by passing the text you want to search as a parameter. For example:

SearchText("example text");

This will search for the text “example text” in the RichTextBox control.