DataGridView AutoFit and Fill

This is my favorite approach...

_dataGrid.DataBindingComplete += (o, _) =>
    {
        var dataGridView = o as DataGridView;
        if (dataGridView != null)
        {
           dataGridView.AutoSizeColumnsMode = DataGridViewAutoSizeColumnsMode.AllCells;
           dataGridView.Columns[dataGridView.ColumnCount-1].AutoSizeMode = DataGridViewAutoSizeColumnMode.Fill;
        }
    };

You need to use the DataGridViewColumn.AutoSizeMode property.

You can use one of these values for column 0 and 1:

AllCells: The column width adjusts to fit the contents of all cells in the column, including the header cell.
AllCellsExceptHeader: The column width adjusts to fit the contents of all cells in the column, excluding the header cell.
DisplayedCells: The column width adjusts to fit the contents of all cells in the column that are in rows currently displayed onscreen, including the header cell.
DisplayedCellsExceptHeader: The column width adjusts to fit the contents of all cells in the column that are in rows currently displayed onscreen, excluding the header cell.

Then you use the Fill value for column 2

The column width adjusts so that the widths of all columns exactly fills the display area of the control...

this.DataGridView1.Columns[0].AutoSizeMode = DataGridViewAutoSizeColumnMode.DisplayedCells;
this.DataGridView1.Columns[1].AutoSizeMode = DataGridViewAutoSizeColumnMode.DisplayedCells;
this.DataGridView1.Columns[2].AutoSizeMode = DataGridViewAutoSizeColumnMode.Fill;

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As pointed out by other users, the default value can be set at datagridview level with DataGridView.AutoSizeColumnsMode property.

this.DataGridView1.Columns[0].AutoSizeMode = DataGridViewAutoSizeColumnMode.DisplayedCells;
this.DataGridView1.Columns[1].AutoSizeMode = DataGridViewAutoSizeColumnMode.DisplayedCells;

could be:

this.DataGridView1.AutoSizeColumnsMode = DataGridViewAutoSizeColumnsMode.DisplayedCells;

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Important note:

If your grid is bound to a datasource and columns are auto-generated (AutoGenerateColumns property set to True), you need to use the DataBindingComplete event to apply style AFTER columns have been created.

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In some scenarios (change cells value by code for example), I had to call DataGridView1.AutoResizeColumns(); to refresh the grid.