Programmatically setting the width of a grid column with * in WPF

You can use auto:

var c1 = new ColumnDefinition() { Width = GridLength.Auto };

Grid grid = new Grid();
ColumnDefinition c1 = new ColumnDefinition();
c1.Width = new GridLength(20, GridUnitType.Star);
ColumnDefinition c2 = new ColumnDefinition();
c2.Width = new GridLength(80, GridUnitType.Star);
grid.ColumnDefinitions.Add(c1);
grid.ColumnDefinitions.Add(c2);

MVVM Approach

View (XAML)

<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="{Binding FirstColumn}"/>
        <ColumnDefinition Width="{Binding SecondColumn}"/>
    </Grid.ColumnDefinitions>
</Grid>

ViewModel (C#)

public class MyViewModel : BindableBase
{
    private GridLength _firstColumn;
    private GridLength _secondColumn;

    public MyViewModel()
    {
        _firstColumn = new GridLength(75, GridUnitType.Star);
        _secondColumn = new GridLength(25, GridUnitType.Star);
    }

    public GridLength FirstColumn
    {
        get { return _firstColumn; }
        set { SetProperty(ref _firstColumn, value); }
    }

    public GridLength SecondColumn
    {
        get { return _secondColumn; }
        set { SetProperty(ref _secondColumn, value); }
    }

    private void NotifyToggleFullScreen(bool isToggleExpansion)
    {
        if (isToggleExpansion)
        {
            FirstColumn = new GridLength(0, GridUnitType.Auto);
            SecondColumn = new GridLength(100, GridUnitType.Star);
        }
        else
        {
            FirstColumn = new GridLength(75, GridUnitType.Star);
            SecondColumn = new GridLength(25, GridUnitType.Star);
        }
    }
}

Suppose you have some buttons (aligned horizontally) in a page and need to hide / show certain ones depending on some status.

<Grid HorizontalAlignment="Center" Grid.Column="1" Width="340" VerticalAlignment="Center" Background="Transparent">

    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="2*" x:Name="colOne"></ColumnDefinition>
        <ColumnDefinition Width="0" x:Name="colTwo"></ColumnDefinition>
        <ColumnDefinition Width="0" x:Name="colThree"></ColumnDefinition>
        <ColumnDefinition Width="0" x:Name="colFour"></ColumnDefinition>
    </Grid.ColumnDefinitions>

    <Button x:Name="btnOne"  Grid.Column="0" Height="50" Width="50" Content="One" Cursor="Hand" />
    <Button x:Name="btnTwo"  Grid.Column="1" Height="50" Width="50" Content="Two" Cursor="Hand" />
    <Button x:Name="btnThree"  Grid.Column="2" Height="50" Width="50" Content="Thre" Cursor="Hand" />
    <Button x:Name="btnFour"  Grid.Column="3" Height="50" Width="50" Content="Four" Cursor="Hand" />

</Grid>

Here btnOne will be visible in the page when executed. btnOne will also be aligned in the center. Now if we want Three and Four to be displayed and One to be hidden when clicked on One, we can use this code:

private void btnOne_Click(object sender, RoutedEventArgs e)
{
    SetGridColWidth(colOne, false);
    SetGridColWidth(colThree, true);
    SetGridColWidth(colFour, true);
}

private void SetGridColWidth(ColumnDefinition column, bool show)
{
    if (show)
        column.Width = new GridLength(2, GridUnitType.Star);
    else
        column.Width = new GridLength(0);
}

You can toggle between visibility of any button at runtime.

Hope this helps someone!