Tutorial :How can I prevent the keydown event of a form in C# from firing more than once?



Question:

According to the official documentation, the KeyDown event on a Windows Forms control occurs only once, but it is easy to demonstrate that the event fires continually aslong as a key is held down:

    private void textBox1_KeyDown(object sender, KeyEventArgs e)      {          label1.Text = string.Format("{0}", globalCounter++);      }  

How can you consume the event so that it fires only once?


Solution:1

I'm generally a VB guy, but this seems to work for me as demo code, using the form itself as the input source:

namespace WindowsFormsApplication1  {      public partial class Form1 : Form      {          private bool _keyHeld;          public Form1()          {              InitializeComponent();              this.KeyUp += new KeyEventHandler(Form1_KeyUp);              this.KeyDown += new KeyEventHandler(Form1_KeyDown);              this._keyHeld = false;          }            void Form1_KeyUp(object sender, KeyEventArgs e)          {              this._keyHeld = false;          }            void Form1_KeyDown(object sender, KeyEventArgs e)          {              if (!this._keyHeld)              {                  this._keyHeld = true;                  if (this.BackColor == Control.DefaultBackColor)                  {                      this.BackColor = Color.Red;                  }                  else                  {                      this.BackColor = Control.DefaultBackColor;                  }              }              else              {                  e.Handled = true;              }          }      }     }  

I think the logic gets a little sketchy if you're holding down multiple keys at a time, but that seems to only fire the event from the last key that was pressed anyway, so I don't think it becomes an issue.

I tested this in a TextBox in VB, and it worked fine. Wasn't sure on the inheritance conventions I should follow in c#, so I left it as a straight Form for this answer.

Apologies for any gross code formatting errors, again, this isn't my usual language.


Solution:2

since the multiple occourrence of KeyDown is due to the keyrepeat settings of Windows, I think that you should somehow track the KeyUp event of that key also to know that the key has been released.


Solution:3

Use KeyUp.


Solution:4

You can override a ProcessCmdKey method.


Solution:5

you could use a counter!


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