WPF xamlreader使用自定义元素加载xaml

Question

I got a canvas with custom elements, similar to texboxes and shapes.

<Canvas Name="SomeCanvas" >
    <TextBox_Element   Canvas.Left="400" Canvas.Top="200" Height="50" Name="s3" Background="#57FF3ACB"  />
</Canvas>

Which im saving to a file with:

SerializeToXML(filename, SomeCanvas);

Later I´m trying to load that file and convert ist back to the original canvas with:

FileStream fs = File.Open(openFileDialog.FileName, FileMode.Open, FileAccess.Read);
Canvas savedCanvas = XamlReader.Load(fs) as Canvas;

The last method throws a XamlParseException:

No matching constructor found on type '...TextBox_Element'. You can use the Arguments or FactoryMethod directives to construct this type.'

Does anybody know how to restore the canvas the way it was from the file? Thanks

Answer 1

You have to define a parameterless constructor in your TextBox_Element .

XamlReader.Load by default will always look for an empty constructor while if it has a required parameters you'll have to add more work.

According to Max Galkin

It's a "feature" of the XAML language, it is declarative and doesn't know anything about constructors. People use ObjectDataProvider in XAML to "translate" and wrap instances of classes that do not have a parameterless constructor (it's [also useful for data binding][2]).

In your case it'll be approximately like this:

<ObjectDataProvider ObjectType="TextBox_Element">
    <ObjectDataProvider.ConstructorParameters>
        <x:Double>400</x:Double>
        <x:Double>200</x:Double>
    </ObjectDataProvider.ConstructorParameters>
</ObjectDataProvider>


var textBoxElem = (TextBox_Element) ((ObjectDataProvider)XamlReader.Parse(xamlStr)).Data;

Assuming your TextBox_Element constructor looks like this

public class TextBox_Element
{
  public TextBox_Element(double left, double top)
  {
    // do whatever
  }
}

Answer 2

Do you have a parameterless constructor in your custom class? If not, try this:

public class TextBox_Element{

    public TextBox_Element(double x, double y){
        // code
    }
    public TextBox_Element(){
        // emptyness
    }
}

That worked for me, i hope itsolves your problem too!

Answer 3

Perhaps the file it saved to did not contain the information required on your custom element? Just as if you save a textbox, the XamlReader needs to have the xmlns references to be able to read it.