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Invalid Cast Exception when updating SvgText Coordinates

See original GitHub issue

The issue is happening when you want to update SvgText.X property value with a new one.

if you run this unit test you will see the error:

[Test]
public void TestUpdateCoordinatesForCollectionChange()
{
    SvgText text = new SvgText()
    {
        Text = "Test coordinates",
        X = { 10 },
        Y = { 30 },
        Dx = { 40 },
        Dy = { 50 }
    };

    text.X = new SvgUnitCollection() { 20 };  // <- This line throws an Invalid Cast Exception

    var xml = text.GetXML();
    Assert.IsTrue(xml.Contains("x=\"20\""));
    Assert.IsTrue(xml.Contains("y=\"30\""));
    Assert.IsTrue(xml.Contains("dx=\"40\""));
    Assert.IsTrue(xml.Contains("dy=\"50\""));
}

Exception detail

System.InvalidCastException
  HResult=0x80004002
  Message=Unable to cast object of type 'Svg.SvgUnitCollection' to type 'Svg.SvgUnit'.
  Source=Svg
  StackTrace:
   at Svg.SvgAttributeCollection.UnboxAndCheck[T](Object a, Object b) in C:\Projects\Svg-Net\SVG\Source\SvgAttributeCollection.cs:line 136
   at Svg.SvgAttributeCollection.TryUnboxedCheck(Object a, Object b) in C:\Projects\Svg-Net\SVG\Source\SvgAttributeCollection.cs:line 118
   at Svg.SvgAttributeCollection.set_Item(String attributeName, Object value) in C:\Projects\Svg-Net\SVG\Source\SvgAttributeCollection.cs:line 99
   at Svg.SvgTextBase.set_X(SvgUnitCollection value) in C:\Projects\Svg-Net\SVG\Source\Text\SvgTextBase.cs:line 70
   at Svg.UnitTests.SvgTextTests.TestWritesCoordinatesForCollectionChange2() in C:\Projects\Svg-Net\SVG\Tests\Svg.UnitTests\SvgTextTests.cs:line 102

From the code reference below you can see this happens in line 70

https://github.com/svg-net/SVG/blob/e37e4441641806205b6aeb055864a899de1adc36/Source/Text/SvgTextBase.cs#L56-L78

We can see the following

From the

_x -> Type = SvgUnitCollection
incoming value -> Type = SvgUnitCollection
Attributes[“x”] -> Type = SvgUnit

So when line 70 is executed it throws an invalid case exception Attributes["x"] = value; .

My observations

It’s valid for a SvgText element to have multiple values for X property as described in MDN page, so passing a collection is valid
From the current code, We can see that it only stores the first element in the collection, see line 76 from referenced code. So I assume multiple values are not supported yet.
As coding practice we may want or not (it depends on what we want to do) overwrite a property that it has a collection, I guess most of the times we don’t want to allow that, but in this case I think it is a good behavior, since you can update a set of values (in the furture) without the need of empty the current collection and then populate by using Collection.Add property, it is not too friendly in this case in my opinion

Proposal

Having said this, I think the SvgText.X property should allow direct collection assignment following the current implementation, that is only supporting one single value for X property by now.

Based on what we have on line 76 I updated line 70 and it worked.

[SvgAttribute("x")]
public virtual SvgUnitCollection X
{
    get { return _x; }
    set
    {
        if (_x != value)
        {
            if (_x != null) _x.CollectionChanged -= OnXChanged;
            _x = value;
            if (_x != null) _x.CollectionChanged += OnXChanged;

            IsPathDirty = true;
        }
        Attributes["x"] = value.FirstOrDefault(); // <- Adding FirstOrDefault sends the SVGUnit value and no cast exception happens
    }
}