Investigate nullability of members in System.Data.Common affected by XmlSerializer.Deserialize

[jozkee]

also IMO we should file an issue to investigate if this should return nullable here and below

Originally posted by @krwq in #41474 (comment)

runtime/src/libraries/System.Data.Common/src/System/Data/Common/ObjectStorage.cs

Line 442 in 8e7a224

retValue = deserializerWithRootAttribute.Deserialize(xmlReader);

runtime/src/libraries/System.Data.Common/src/System/Data/Common/ObjectStorage.cs

Line 344 in 8e7a224

return (deserializerWithOutRootAttribute.Deserialize(strreader))!;

runtime/src/libraries/System.Data.Common/src/System/Data/Common/SqlUDTStorage.cs

Line 164 in 8e7a224

return (deserializerWithOutRootAttribute.Deserialize(strreader))!;

runtime/src/libraries/System.Data.Common/src/System/Data/Common/SqlUDTStorage.cs

Line 191 in 8e7a224

return (deserializerWithRootAttribute.Deserialize(xmlReader))!;

[ghost]

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[roji]

I took a quick look at this, and non-nullable seems like the right annotation (though I'm not very familiar with this part of the code base). For example, XMLDiffLoader.ReadOldRowData calls DataColumn.ConvertXmlToObject (the method in question), and assigns its results to DataColumn[int], which is clearly non-nullable (there's a Debug.Assert check for that).

In any case, DataStorage.cs (where this method resides) is internal, and the nullability here doesn't seem to leak anywhere close to public surface APIs, so the importance here is limited and we can always fix in the future.

So I'll go ahead and close this.