AVRO-3110: Infinite Recursion - CSharp

[eaba]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Avro Jira issues and references them in the PR title. "AVRO-3110: Infinite Recursion"
  -https://issues.apache.org/jira/browse/AVRO-3110).

Tests

• [ x] TestRecursiveSchema: https://github.com/eaba/AvroSchemaGenerator/blob/nested_schema/AvroSchemaGenerator.Tests/GetSchemaTest.cs

Commits

• My commits all reference Jira issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

[blachniet]

Do you think you could add a unit test that verifies this fix?

In other words, a test that fails before the changes, but passes afterwards.

[blachniet]

I believe you can collapse the omit the ContainsKey by checking the return value of TryAdd. The docs for TryAdd state that it returns:

true if the key/value pair was added to the ConcurrentDictionary<TKey,TValue> successfully; false if the key already exists.

So, maybe you can do something like this:

Suggested change

	if (!_previousFields.ContainsKey(f.Name))
	{
	_previousFields.TryAdd(f.Name, f.Schema);
	if (_previousFields.TryAdd(f.Name, f.Schema);)
	{

[eaba]

Do you think you could add a unit test that verifies this fix?

In other words, a test that fails before the changes, but passes afterwards.

@blachniet I have added test case