Some useful TextBox Validations

Numeric TextBox

private void textBox1_KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
{
if ( !( char.IsDigit( e.KeyChar ) || char.IsControl( e.KeyChar ) ) )
{
e.Handled = true;
}
}

Numeric TextBox with Decimals

private void textBox1_KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
{
if ( !( char.IsDigit( e.KeyChar) || char.IsControl( e.KeyChar ) ||(e.KeyChar== (char )46)) )
{
e.Handled = true;
}
}

TextBox Allowing Characters Only

private void textBox1_KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
{
if ( !( char.IsLetter( e.KeyChar ) || char.IsControl( e.KeyChar ) ) )
{
e.Handled = true;
}
}

TextBox Allowing Upper Case Characters Only

private void textBox1_KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
{
if ( !( char.IsUpper( e.KeyChar ) || char.IsControl( e.KeyChar )) )
{
e.Handled = true;
}
}

TextBox Allowing Lower Case Characters Only

private void textBox1_KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
{
if ( !( char.IsLower( e.KeyChar ) || char.IsControl( e.KeyChar )) )
{
e.Handled = true;
}
}

Check For Unfilled TextBox

// Call this function and pass the Textbox as parameter to this function
public static bool ChkEmpty(params System.Windows.Forms.TextBox[ ] tb)
{
int i;
for (i = 0; i < tb.Length; i++)
{
if (tb[i].Text.Trim() == "")
{
MessageBox.Show("Don't keep field empty");
tb[i].Focus();
return false;
}
}
return true;
}

http://www.codeproject.com/useritems/tips.asp#tip3.4.3