Wpf – binding stringformat on label using string literal

string-formattingwpfxaml

I've binded the tooltip of a slider control to it's Value property and i'm trying to use StringFormat to make it display "Current Value {0} of 10" where the {0} is the Value property. Below is one of the various things I tried when trying to figure this out.

<Slider.ToolTip>
  <Label>
    <Label.Content>
      <Binding StringFormat="Current Value {0} of 10"
               ElementName="DebugLevelSlider"
               Path="Value" />
    </Label.Content>
  </Label>
</Slider.ToolTip>

I am having issues finding examples online of how to use stringformat with string literals such as mine above. I see a lot of stringformat date/time/currency format conversion. I think I have a way to do this with a multibinding but it just seems like an extra amount of work than necessary. I hope that for string literal formatting I still do not have to write a custom converter.

In my app I find myself using a lot of extra labels next to items so getting an understanding in the stringformat will hopefully let me eliminate some of those unnecessary labels.

Best Answer

Label.Content is object so you can't use Binding.StringFormat there as the binding's target type must be string in order for it to work.

Two work arounds are: use TextBlock instead of Label and bind the Text property.

Use Label.ContentStringFormat i.e.

<Label ContentStringFormat="Current Value {0} of 10" Content={Binding ...} />

You only need to escape the string with {} if your first character is a {

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WPF binding StringFormat syntax

The extra 0 comes from the 0 after the colon.

Instead, try {}{0:#,0}.

From the MSDN docs on Custom Numeric String formats (emphasis added):

"0" | Zero placeholder | Replaces the zero with the corresponding digit if one is present; otherwise, zero appears in the result string.

WPF StringFormat on Label Content

The reason this doesn't work is that the Label.Content property is of type Object, and Binding.StringFormat is only used when binding to a property of type String.

What is happening is:

The Binding is boxing your MaxLevelOfInvestment value and storing it the Label.Content property as a boxed decimal value.
The Label control has a template that includes a ContentPresenter.
Since ContentTemplate is not set, ContentPresenter looks for a DataTemplate defined for the Decimal type. When it finds none, it uses a default template.
The default template used by the ContentPresenter presents strings by using the label's ContentStringFormat property.

Two solutions are possible:

Use Label.ContentStringFormat instead of Binding.StringFormat, or
Use a String property such as TextBlock.Text instead of Label.Content

Here is how to use Label.ContentStringFormat:

<Label Content="{Binding Path=MaxLevelofInvestment}" ContentStringFormat="Amount is {0}" />

Here is how to use a TextBlock:

<TextBlock Text="{Binding Path=MaxLevelofInvestment, StringFormat='Amount is {0}'}" />

Note: For simplicity I omitted one detail in the above explanation: The ContentPresenter actually uses its own Template and StringFormat properties, but during loading these are automatically template-bound to the ContentTemplate and ContentStringFormat properties of the Label, so it seems as if the ContentPresenter is actually using the Label's properties.

Best Answer

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