This works:
<PictureBox Width="4.76060201236516cm" Height="2.80000019073486cm" Left="4.40019964058639cm" Top="5.5cm" Sizing="ScaleProportional" MimeType="image/png" Name="companyLogoPictureBox" Value="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">
<Style TextAlign="Center" VerticalAlign="Middle" />
</PictureBox>
But this does not:
<PictureBox Width="4.76060201236516cm" Height="2.80000019073486cm" Left="4.40019964058639cm" Top="5.5cm" Sizing="ScaleProportional" MimeType="image/png" Name="companyLogoPictureBox" Value="= IsNull(Fields.CollectorSignature)">
<Style TextAlign="Center" VerticalAlign="Middle" />
</PictureBox>
My Table stored the field "CollectorSignature" as VARHCAR(MAX) and contains :
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
I get Error:
Value cannot be null.
Parameter name: key