How can I define a variable in XAML?

I have the following two buttons in XAML:

<Button Content="Previous"
        Margin="10,0,0,10"/>
<Button Content="Next"
        Margin="0,0,10,10"/>

How can I define "10" to be a variable so I can change it in one place, something like this:

PSEUDO CODE:

<variable x:key="theMargin"/>
<Button Content="Previous"
        Margin="{Variable theMargin},0,0,{Variable theMargin}"/>
<Button Content="Next"
        Margin="0,0,{Variable theMargin},{Variable theMargin}"/>

Try this:

add to the head of the xamlfile

xmlns:System="clr-namespace:System;assembly=mscorlib"

Then Add this to the resource section:

<System:Double x:Key="theMargin">2.35</System:Double>

Lastly, use a thickness on the margin:

<Button Content="Next">
   <Button.Margin>
      <Thickness Top="{StaticResource theMargin}" Left="0" Right="0"
                  Bottom ="{StaticResource theMargin}" />
   </Button.Margin>
</Button>

A lot of system types can be defined this way: int, char, string, DateTime, etc

Note: You're right... Had to do some better testing.. changed to code so that it should work

Similiar to Sorskoot's answer, you can add a thickness resource to use, thereby defining each margin direction independently

<UserControl.Resources>
    <Thickness x:Key="myMargin" Top="5" Left="10" Right="10" Bottom ="5"></Thickness>
</UserControl.Resources>

Then just use the Thickness as the Margin:

<Button Content="Next" Margin="{StaticResource myMargin}"/>

Why don't you try adding the value as a StaticResource?

Resources.Add("theMargin", 10);

Then you can get that value like this:

<Button Content="Previous"
        Margin="{StaticResource theMargin},0,0,{StaticResource theMargin}"/>
<Button Content="Next"
        Margin="0,0,{StaticResource theMargin},{StaticResource theMargin}"/>

You need invoke this before InitializeComponent or use INotifyPropertyChanged Interface after that

In .NET Core @Sorskoot's answer was working, but produced a warning. So instead I used this namespace declaration:

xmlns:system="clr-namespace:System;assembly=System.Runtime"