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InvalidCastException in PdfAnnotations
https://forum.pdfsharp.net/viewtopic.php?f=3&t=1197		 Page 1 of 1
Author:  RichardD [ Thu May 27, 2010 12:10 pm ]
Post subject:  InvalidCastException in PdfAnnotations
Affects: PDFSharp v1.31.1789.0, both GDI+ build and WPF build;

Attempting to iterate a PdfAnnotations instance with a foreach loop results in an InvalidCastException:
Code:
foreach (var annotation in page.Annotations)
{
    ...
}

Quote:
Unable to cast object of type 'AnnotationsIterator' to type 'System.Collections.Generic.IEnumerator`1[PdfSharp.Pdf.PdfItem]'
at PdfSharp.Pdf.Annotations.PdfAnnotations.GetEnumerator()



In v1.2, the PdfArray class implemented IEnumerable, and the nested AnnotationsIterator class implemented IEnumerator; everything worked as expected.

Code:
public sealed class PdfAnnotations : PdfArray
{
    ...
    public override IEnumerator GetEnumerator()
    {
        return new AnnotationsIterator(this);
    }
   
    class AnnotationsIterator : IEnumerator { ... }
}



In v1.31, PdfArray implements IEnumerable<PdfItem>, and AnnotationsIterator implements IEnumerator<PdfAnnotation>. The PdfAnnotations.GetEnumerator method creates a new instance of the AnnotationsIterator, and attempts to cast it to IEnumerator<PdfItem>. This would work in .NET 4.0, but fails in 2.0, 3.0 or 3.5, as generic covariance is not available.

Code:
public sealed class PdfAnnotations : PdfArray
{
    ...
    public override IEnumerator<PdfItem> GetEnumerator()
    {
        return (IEnumerator<PdfItem>)new AnnotationsIterator(this);
    }
   
    class AnnotationsIterator : IEnumerator<PdfAnnotation> { ... }
}



To fix this, the AnnotationsIterator class needs to implement IEnumerator<PdfItem> instead of IEnumerator<PdfAnnotation>. The PdfAnnotations.GetEnumerator method will then work without the cast.

Code:
public sealed class PdfAnnotations : PdfArray
{
    ...
    public override IEnumerator<PdfItem> GetEnumerator()
    {
        return new AnnotationsIterator(this);
    }
   
    class AnnotationsIterator : IEnumerator<PdfItem> { ... }
}



The only workaround for v1.31 is to iterate manually:
Code:
for (int index = 0; index < page.Annotations.Count; index++)
{
    var annotation = page.Annotations[index];
    ...
}
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