[alaricd]

Summary

Implement ranged character reads for Spanner STRING values, with length queries when the buffer is null. Reject null and non-STRING values with InvalidCastException, and add tests for ranged reads and these cases.

Resolves #16042.

Microsoft contract

Microsoft documents DbDataReader.GetChars as reading a requested character range from a column into a buffer: https://learn.microsoft.com/dotnet/api/system.data.common.dbdatareader.getchars. Its DataTableReader implementation documents InvalidCastException when the column does not contain character data and accepts nullable buffers: https://learn.microsoft.com/dotnet/api/system.data.datatablereader.getchars

Compatibility

This replaces NotSupportedException for Spanner STRING values with the documented read behavior. Other value types and SQL NULL values report InvalidCastException; callers that depended on the previous unsupported exception for strings will observe the new supported result.

Validation

dotnet test apis/Google.Cloud.Spanner.Data/Google.Cloud.Spanner.Data.Tests/Google.Cloud.Spanner.Data.Tests.csproj --no-restore -f net8.0 -p:NoWarn=CS8981%3BxUnit2004%3BxUnit2013 — 2,834 passed, 0 failed.

Discovery

Found using pengdows.AdoNetApiTest.conformance.

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[gemini-code-assist]

Code Review

This pull request implements the GetChars method in SpannerDataReader to support reading string data into character arrays, along with corresponding unit tests. A review comment suggests simplifying and correcting the buffer range validation checks to prevent misleading exception messages and improve clarity.

[gemini-code-assist]

The validation checks for length and bufferOffset + length can be simplified and made more precise. Currently, checking bufferOffset + length using nameof(length) as the parameter name can produce misleading exception messages (e.g., showing the sum of offset and length as the invalid 'length' value). By checking length against buffer.Length - bufferOffset, we can ensure that the requested length fits within the remaining buffer space. This mathematically guarantees that bufferOffset + length <= buffer.Length without needing a third check, and provides a much clearer error message if the validation fails.

            GaxPreconditions.CheckArgumentRange(bufferOffset, nameof(bufferOffset), 0, buffer.Length);
            GaxPreconditions.CheckArgumentRange(length, nameof(length), 0, buffer.Length - bufferOffset);