feat: BigQuery Storage v1beta1 API migration guide

[benhxy]

Fixes b/505001153. BigQuery Storage team is planning to deprecate v1beta1 API. This PR adds a migration guide from v1beta1 to v1.

[gemini-code-assist]

Code Review

This pull request introduces a migration guide for transitioning the BigQuery Storage API from v1beta1 to v1 in C#. The feedback focuses on making the C# code examples more idiomatic and robust. Key suggestions include using the strongly-typed TableName class instead of a raw string path, clarifying how ReadRowsStream relates to IAsyncEnumerable, and using var to avoid type ambiguity with System.IO.Stream.

[gemini-code-assist]

While you can use a string path for Table, Google Cloud .NET libraries provide a strongly-typed TableName class and a TableAsTableName property on ReadSession. This is more idiomatic, less error-prone, and matches the quickstart sample in this repository.

Suggested change

	// Table path is now a string: projects/{project}/datasets/{dataset}/tables/{table}
	string tablePath = "projects/bigquery-public-data/datasets/usa_names/tables/usa_1910_current";
	ReadSession.Types.TableReadOptions readOptions = new ReadSession.Types.TableReadOptions();
	readOptions.SelectedFields.Add("name");
	readOptions.RowRestriction = "state = 'WA'";
	// ReadSession holds the session configuration
	ReadSession readSession = new ReadSession
	{
	Table = tablePath,
	DataFormat = DataFormat.Avro, // Format renamed to DataFormat
	ReadOptions = readOptions
	};
	// Table path can be a string or a strongly-typed TableName
	var tableName = new Google.Api.Gax.ResourceNames.TableName("bigquery-public-data", "usa_names", "usa_1910_current");
	ReadSession.Types.TableReadOptions readOptions = new ReadSession.Types.TableReadOptions();
	readOptions.SelectedFields.Add("name");
	readOptions.RowRestriction = "state = 'WA'";
	// ReadSession holds the session configuration
	ReadSession readSession = new ReadSession
	{
	TableAsTableName = tableName,
	DataFormat = DataFormat.Avro, // Format renamed to DataFormat
	ReadOptions = readOptions
	};

[gemini-code-assist]

In C#, ReadRowsStream does not implement IAsyncEnumerable<T> directly. Instead, you call GetResponseStream() on it, which returns an AsyncResponseStream<T> that implements IAsyncEnumerable<T>. We should clarify this to avoid confusion.

Suggested change

	In C#, `ReadRows` returns a `ReadRowsStream` which implements `IAsyncEnumerable`
	or similar, allowing async iteration.
	In C#, ReadRows returns a ReadRowsStream. Calling GetResponseStream() on it returns an AsyncResponseStream which implements IAsyncEnumerable, allowing async iteration.

[gemini-code-assist]

Using Stream as an explicit type name can cause ambiguity with System.IO.Stream, which is very commonly imported in C# files dealing with data streams. Using var is more idiomatic and avoids namespace collisions.

Suggested change

	Stream stream = session.Streams[0];
	var stream = session.Streams[0];

[gemini-code-assist]

Similarly, using var here is more idiomatic and consistent with modern C# style guidelines, while also avoiding any potential namespace collisions.

Suggested change

	ReadStream stream = session.Streams[0];
	var stream = session.Streams[0];