How to select default button in wpf dialog?

How about IsDefault?

<Button Command="{Binding MyButtonCommand}"
        Content="My Button Text"
        IsDefault="True"/>

To set a Window's Default button

Set your default button's IsDefault property to true.

Note that you can also set a Window's Cancel button by setting a button's IsCancel property to true.

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To set the Selected (focused) button in a Window

If you want to select a particular button then use the Focus method like this:

yourButton.Focus();

You might do this when a Window loads (in the Window_Loaded event).

To select a particular button when your Window opens make sure its IsTabStop property is set to true and ensure its TabIndex property is lower than any other control on the Window.

just create a new button template and change the look and feel for the IsDefault=Tue state. I just created a style and modified the state.

<Window
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:System="clr-namespace:System;assembly=mscorlib"
xmlns:Microsoft_Windows_Themes="clr-namespace:Microsoft.Windows.Themes;assembly=PresentationFramework.Aero"
x:Class="WpfApplication7.Window3"
x:Name="Window"
Title="Window3"
Width="640" Height="480" FocusManager.FocusedElement="{Binding ElementName=test}">

<Window.Resources>
    <Style x:Key="ButtonFocusVisual">
        <Setter Property="Control.Template">
            <Setter.Value>
                <ControlTemplate>
                    <Rectangle Margin="2"
                    Stroke="red" StrokeThickness="1"
                     SnapsToDevicePixels="true"  StrokeDashArray="1 2"/>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

    <Style x:Key="ButtonStyle1" TargetType="{x:Type Button}">
        <Setter Property="FocusVisualStyle" Value="{StaticResource ButtonFocusVisual}"/>
        <Setter Property="HorizontalContentAlignment" Value="Center"/>
        <Setter Property="VerticalContentAlignment" Value="Center"/>
        <Setter Property="Padding" Value="1"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type Button}">
                    <Border x:Name="border" BorderThickness="1" BorderBrush="#FF040000" CornerRadius="5">
                        <Border.Background>
                            <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
                                <GradientStop Color="#FF7A7A7A" Offset="0"/>
                                <GradientStop Color="#FFE7E7E7" Offset="1"/>
                            </LinearGradientBrush>
                        </Border.Background>
                        <ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
                    </Border>
                    <ControlTemplate.Triggers>
                                                    <Trigger Property="IsDefault" Value="True">
                            <Setter Property="Background" TargetName="border">
                                <Setter.Value>
                                    <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
                                        <GradientStop Color="#FFA76F6F" Offset="0"/>
                                        <GradientStop Color="#FFE7E7E7" Offset="1"/>
                                    </LinearGradientBrush>
                                </Setter.Value>
                            </Setter>
                        </Trigger>
                        <MultiTrigger>
                            <MultiTrigger.Conditions>
                                <Condition Property="IsFocused" Value="True"/>
                                <Condition Property="IsDefault" Value="True"/>
                            </MultiTrigger.Conditions>
                            <Setter Property="Background" TargetName="border">
                                <Setter.Value>
                                    <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
                                        <GradientStop Color="#FFC2BE5B" Offset="0.007"/>
                                        <GradientStop Color="#FFE7E7E7" Offset="1"/>
                                    </LinearGradientBrush>
                                </Setter.Value>
                            </Setter>
                        </MultiTrigger>
                        <Trigger Property="IsPressed" Value="True">
                            <Setter Property="BorderBrush" TargetName="border" Value="#FF01641D"/>
                            <Setter Property="BorderThickness" TargetName="border" Value="2"/>
                            <Setter Property="Background" TargetName="border">
                                <Setter.Value>
                                    <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
                                        <GradientStop Color="#FF528159" Offset="0"/>
                                        <GradientStop Color="#FFE7E7E7" Offset="1"/>
                                    </LinearGradientBrush>
                                </Setter.Value>
                            </Setter>
                        </Trigger>

                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</Window.Resources>

<Grid x:Name="LayoutRoot">
    <Button x:Name="test" Content="Button" HorizontalAlignment="Left" Height="26" Margin="130,157,0,0" VerticalAlignment="Top" Width="164" Style="{DynamicResource ButtonStyle1}" IsDefault="True"/>
    <Button Content="Button" Height="26" Margin="298,157,162,0" VerticalAlignment="Top" Style="{DynamicResource ButtonStyle1}"/>
    <Button Content="Button" HorizontalAlignment="Right" Height="26" Margin="0,157,-6,0" VerticalAlignment="Top" Width="164" Style="{DynamicResource ButtonStyle1}"/>
</Grid>