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[PRB] Invalid assignments to the Rtf property might not throw an exception

If an invalid RTF string is assigned to the Rtf property of the VB6 RichTextBox control, value is usually regarded as plain text and assigned to the Text property. However, this behavior isn’t fully consistent; for example, it may happen that curly braces embedded in the string are discarded. You can prove this by executing the following VB6 code:

        ' Rtf1 is a RichTextBox control
        Me.Rtf1.Rtf = "Test String"    ' should raise an error, but it doesn’t
        Me.Rtf1.Rtf = "Text {0}"       ' preserves curly braces, displays "Text {0}"
        Me.Rtf1.Rtf = "{Text}"         ' discards curly braces, displays "Text"

By contrast, the .NET RichTextBox control has a more predictable behavior: when an invalid RTF string is assigned to the Rtf property a runtime exception is thrown. The VB6RichTextBox control adopts the following compromise: it attempts to assign the Rtf property and, if the assignment fails, the value is assigned to the Text property. In other words, an assignment to the Rtf property can never raise an error in an application that has been converted to VB.NET.

However, some VB6 application might require that an error be raised for invalid RTF strings. For example, this behavior is desirable if the application has to validate an RTF string. In these cases, you can assign True to the ErrorOnInvalidRtf Boolean property that the VB6RichTextBox control exposes. You can set this property by means of an InsertStatement pragma or a WriteProperty pragma:

        '## Rtf1.WriteProperty ErrorOnInvalidRtf, True
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