Enable / Disable Button according to Checkbox value in a DataGridView

You can do this by using the CellValueChanged event like this:

private void dataGridView1_CellValueChanged(object sender, DataGridViewCellEventArgs e)
{
    // se a célula alterada for a célula de interesse (1.)
    if (dataGridView1.Columns[e.ColumnIndex].Name.Equals("chk"))
    {
        bool marcado = false;
        foreach (DataGridViewRow row in dataGridView1.Rows)
        {
            DataGridViewCheckBoxCell chk = (DataGridViewCheckBoxCell)row.Cells["chk"]; // (2.)
            // se a célula estiver marcada
            if ((bool)(chk.Value ?? false) == true) // (3.) e (4.)
            {
                marcado = true;
                break;
            }
        }

        if (marcado)
            button1.Enabled = true;
        else
            button1.Enabled = false;
    }
}

Here are a few things to keep in mind:

I'm not sure exactly how much data you'll be working on, but doing this for every time a cell is checked / unchecked, you probably will not perform well, I thought of another solution, you might need to better, but it already serves as a starting point.

Create a global variable private int numeroCelulasMarcadas = 0; , this variable will serve as a counter of marked cells, in event RowsAdded of DataGridView , do so:

private void dataGridView1_RowsAdded(object sender, DataGridViewRowsAddedEventArgs e)
{
    DataGridViewCheckBoxCell chk =
        (DataGridViewCheckBoxCell)dataGridView1.Rows[e.RowIndex].Cells["chk"];

    if ((bool)(chk.Value ?? false) == true)
    {
        numeroCelulasMarcadas++;
    }
}

Now in event CellValueChanged put this:

private void dataGridView1_CellValueChanged(object sender, DataGridViewCellEventArgs e)
{
    // se a célula alterada for a célula de interesse
    if (dataGridView1.Columns[e.ColumnIndex].Name.Equals("chk"))
    {
        DataGridViewCheckBoxCell chk = 
            (DataGridViewCheckBoxCell)dataGridView1.Rows[e.RowIndex].Cells["chk"];

        if ((bool)chk.Value == true)
        {
            numeroCelulasMarcadas++;
        }
        else
        {
            numeroCelulasMarcadas--;
        }

        if (numeroCelulasMarcadas == 0)
        {
            button1.Enabled = false;
        }
        else
        {
            button1.Enabled = true;
        }
    }
}

Remember that this last solution is a starting point, it may be necessary to adjust something to work correctly in the context of your application.

The solution that Matthew proposed works, but you have to press Enter , in my case the user just wanted to click on CheckBox , so I used the following solution:

private void dataGridView1_CellContentClick(object sender, DataGridViewCellEventArgs e)
{
    if (dataGridView1.Columns[e.ColumnIndex].Name.Equals("chk"))
    {
        ManipulaBotao();
    }
}

private void dataGridView1_CellContentDoubleClick(object sender, DataGridViewCellEventArgs e)
{
    if (dataGridView1.Columns[e.ColumnIndex].Name.Equals("chk"))
    {
        ManipulaBotao();
    }
}

private void ManipulaBotao()
{
    button1.Enabled = false;
    foreach (DataGridViewRow dr in dataGridView1.Rows)
    {
        if (bool.Parse(dr.Cells["chk"].EditedFormattedValue.ToString()))
        {
            button1.Enabled = true;
            break;
        }
    }
}

So just by clicking on CheckBox the control is enabled / disabled.

It's worth mentioning that in my case the number of lines is small so it does not affect the performance, as was well remembered in the post of Matthew.

Note the use of the CellContentDoubleClick method, otherwise it will not work as expected.