[C#] Issues with the C data interface implementation.

[teo-tsirpanis]

Describe the bug, including details regarding any error messages, version, and platform.

I took a look at the code in the Apache.Arrow.C namespace and noticed the following things:

• The native callbacks can use [UnmanagedCallersOnly] on supported frameworks (.NET 5+). This way, taking the pointer to them will be more efficient and does not involve delegates.
• We are manually zeroing the structs we just allocated (

  arrow/csharp/src/Apache.Arrow/C/CArrowArray.cs

  Lines 54 to 63 in 05fe0d2

  	ptr->length = 0;
  	ptr->n_buffers = 0;
  	ptr->offset = 0;
  	ptr->buffers = null;
  	ptr->n_children = 0;
  	ptr->children = null;
  	ptr->dictionary = null;
  	ptr->null_count = 0;
  	ptr->release = null;
  	ptr->private_data = null;

  for example). We can zero them by just writing *ptr = default which is more efficient.
• When releasing an exported array stream we don't free the GCHandle to the managed object, causing a memory leak.
• When a function on an imported array fails, we don't include the result of get_last_error in the exception message.
• We free the last exported array stream error message with Marshal.AllocCoTaskMem, while we had allocated it with Marshal.AllocHGlobal.

  arrow/csharp/src/Apache.Arrow/C/CArrowArrayStreamExporter.cs

  Line 173 in 05fe0d2

  Marshal.FreeCoTaskMem((IntPtr)LastError);

• And more smaller stuff.

I will open a PR to address them.

Component(s)

C#