WPF buttons same/recommended width

Another, perhaps simpler, way to do this is to use the SharedSizeGroup property on the ColumnDefinition and RowDefinition classes.

Columns (and Rows) in a WPF Grid can automatically resize to fit their contents - when SharedSizeGroup is used, columns with the same group name share their resizing logic.

The Xaml would look something like this ...

<Grid Grid.IsSharedSizeScope="True">

    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="*" />
        <ColumnDefinition SharedSizeGroup="Buttons" />
        <ColumnDefinition SharedSizeGroup="Buttons" />
        <ColumnDefinition SharedSizeGroup="Buttons" />
    </Grid.ColumnDefinitions>

    <Grid.RowDefinitions>
        <RowDefinition Height="Auto" />
        <RowDefinition Height="*" />
    </Grid.RowDefinitions>

    <Button Grid.Column="1"
            HorizontalAlignment="Stretch"
            VerticalAlignment="Center"
            Content="Ok"
            Margin="4" />

    <Button Grid.Column="2"
            HorizontalAlignment="Stretch"
            VerticalAlignment="Center"
            Content="Cancel"
            Margin="4" />

    <Button Grid.Column="3"
            HorizontalAlignment="Stretch"
            VerticalAlignment="Center"
            Content="Long Button Caption"
            Margin="4" />
</Grid>

There are several ways to do this:

1) Use a Grid for layout. Each Button gets its own Column, which is Star-sized. That way, all columns are the same size:

<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="*" />
        <ColumnDefinition Width="*" />
        <ColumnDefinition Width="*" />
    </Grid.ColumnDefinitions>
    <Button Grid.Column="0">Yes</Button>
    <Button Grid.Column="1">No</Button>
    <Button Grid.Column="2">Cancel</Button>
</Grid>

2) You can have one item as "master size" and bind the width of all others to this item's width.

<StackPanel Orientation="Horizontal">
    <Button Name="MasterButton" Width="100">Yes</Button>
    <Button>
        <Button.Width>
            <Binding ElementName="MasterButton" Path="Width"/>
        </Button.Width>
        No
    </Button>
</StackPanel>

EDIT: In actual code, you probably will have Width="Auto". Since the other widths are based on the "master width", the button with the widest width (widest text) should be chosen.

Use a "master" control, like in Daniel's answer, but bind to the "ActualWidth" attribute instead of "Width":

<StackPanel Orientation="Horizontal">
    <Button Name="MasterButton">Yes</Button>
    <Button>
        <Button.Width>
            <Binding ElementName="MasterButton" Path="ActualWidth"/>
        </Button.Width>
        No
    </Button>
</StackPanel>

This way, the value is taken from the master control at run time, after minimum and maximum width and all other layout calculations have been taken into account. Binding to "Width" binds to whatever you happen to put into the attribute at compile time, which may not be the width that is really used.

Also, the binding can be written shorter like

<Button Width="{Binding ElementName=MasterButton, Path=ActualWidth}"/>