Question

I’ve got this code in a custom class derived from DataGridView:

protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
{
    if (char.IsNumber(Convert.ToChar(keyData)) || 
        char.IsControl(Convert.ToChar(keyData)) || 
        (keyData >= Keys.NumPad0 && keyData <= Keys.NumPad9) ||
        (keyData == Keys.Up) ||
        (keyData == Keys.Down) ||
        (keyData == Keys.Left) ||
        (keyData == Keys.Right) ||
        (keyData == Keys.Home) ||
        (keyData == Keys.PageDown) ||
        (keyData == Keys.PageUp) ||
        (keyData == Keys.Space) ||
        (keyData == Keys.Back) ||
        (keyData == Keys.Decimal))
    {
        return false;
    }
    return true;
}

I can enter data (numbers and .), and tab from cell to cell, but if I hit the “Shift” key, I get, “System.OverflowException was unhandled by user code
Message=Value was either too large or too small for a character.
Source=mscorlib
StackTrace:
at System.Convert.ToChar(Int32 value)…”

I assume this is the line that’s causing the problem:

char.IsControl(Convert.ToChar(keyData))

…but why is Shift problematic, and what should I do to get it to disregard the Shift key (there’s no reason for the user to press the Shift key in the DGV).

UPDATE

I must admit I don’t quite understand quetzalcoatl’s answer, but I tried to apply it this way:

Keys specials = keyData & Keys.Modifiers; 
Keys keycode = keyData & ~Keys.Modifiers;

if (char.IsNumber(Convert.ToChar(keyData)) || 
    //char.IsControl(Convert.ToChar(keyData)) || 
    //(keyData == specials) || <-- didn't work
    //(keyData == keycode) ||  <-- didn't work
    (keyData != specials) ||
    (keyData != keycode) ||
    . . .

…and still get the same error. How can I apply the information he provided to solve the problem?

UPDATE 2

I’m still struggling with this.

I’ve tried this:

protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
{
    Keys keycode = keyData & ~Keys.Modifiers;

    if (keycode >= Keys.D0 && keycode <= Keys.D9 ||          
        keycode >= Keys.NumPad0 && keycode <= Keys.NumPad9 ||
        (keycode == Keys.Up) ||
        (keycode == Keys.Down) ||
        (keycode == Keys.Left) ||
        (keycode == Keys.Right) ||
    (keyData == Keys.Tab) ||
        (keycode == Keys.Tab) ||
        (keycode == Keys.Home) ||
        (keycode == Keys.PageDown) ||
        (keycode == Keys.PageUp) ||
        (keycode == Keys.Space) ||
        (keycode == Keys.Back) ||
        (keycode == Keys.Decimal))
    {
        return false;
    }
    return true;
}

…and this approach:

protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
        Keys keycode = keyData & ~Keys.Modifiers;

        bool isDigit = keycode >= Keys.D0 && keycode <= Keys.D9 ||
                       keycode >= Keys.NumPad0 && keycode <= Keys.NumPad9;
        bool isControl = (keyData & Keys.Modifiers) != Keys.None;

        if (isDigit ||
            isControl ||
            (keyData == Keys.Up) ||
            (keyData == Keys.Down) ||
            (keyData == Keys.Left) ||
            (keyData == Keys.Right) ||
        (keyData == Keys.Tab) ||
            (keyData == Keys.Home) ||
            (keyData == Keys.PageDown) ||
            (keyData == Keys.PageUp) ||
            (keyData == Keys.Space) ||
            (keyData == Keys.Back) ||
            (keyData == Keys.Decimal))
        {
            return false;
        }
        return true;
}

Both of them solve the Shift key causing an overflow, but both of them prevent a decimal from being entered. I don’t understand that, as Decimal is being (supposedly) explicitly allowed. Everything else explicitly checked for is allowed (Home, PageUp, PageDown, etc.) – it’s just the “.” character that is being barred entry.

UPDATE 3

Using the second approach above and replacing “Keys.Decimal” with “Keys.OemPeriod” works:

protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
{
    Keys keycode = keyData & ~Keys.Modifiers;

    bool isDigit = keycode >= Keys.D0 && keycode <= Keys.D9 ||
                   keycode >= Keys.NumPad0 && keycode <= Keys.NumPad9;
    bool isControl = (keyData & Keys.Modifiers) != Keys.None;

    if (isDigit ||
        isControl ||
        (keyData == Keys.Up) ||
        (keyData == Keys.Down) ||
        (keyData == Keys.Left) ||
        (keyData == Keys.Right) ||
        (keyData == Keys.Home) ||
        (keyData == Keys.Tab) ||
        (keyData == Keys.PageDown) ||
        (keyData == Keys.PageUp) ||
        (keyData == Keys.Space) ||
        (keyData == Keys.Back) ||
        (keyData == Keys.OemPeriod))
    {
        return false;
    }
    return true;
}

Answer 1

Do not convert a Keys to char. This does not work. Test if it is a digit with

Keys code = keyData & ~Keys.Modifiers;

bool isDigit = code >= Keys.D0 && code <= Keys.D9 ||
               code >= Keys.NumPad0 && code <= Keys.NumPad9

Test if it is a control key with

bool isControl = (keyData & Keys.Modifiers) != Keys.None;