CSharp bindings: user-defined string encoding and improved safety

[Mbucari]

What does this PR do?

Creates an interface for users to create custom string Encoder/Decoders for Gdal to use for all string operations. This serves a purpose similar to #3825 by allowing users to manage how strings are encoded/decoded to/from Gdal.

Expose I$moduleStringEncoder interfaces which have 2 methods:

• string FromNullTerminated(IntPtr pStr); - Read native null-terminated data from a Gdal pointer and return a managed string.
• byte[] ToNullTerminated(string str); - Create a null-terminated array of bytes from a managed string to send to Gdal.

It also simplifies string marshalling changes made in recent PR by no longer requires C# to allocate/free any unmanaged memory. All string/byte[] objects are managed.

---

Here's an example of a custom StringEncoder.

class CustomGdalEncoder : OSGeo.GDAL.IGdalStringEncoder, OSGeo.OGR.IOgrStringEncoder, OSGeo.OSR.IOsrStringEncoder
{
    public string FromNullTerminated(IntPtr pStr)
      => Marshal.PtrToStringUTF8(pStr);

    public byte[] ToNullTerminated(string str)
    {
        if (str == null)
            return null;
        int byteCount = Encoding.UTF8.GetByteCount(str);
        var bts = new byte[byteCount + 1];
        Encoding.UTF8.GetBytes(str, 0, str.Length, bts, 0);
        return bts;
    }
}
var encoder = new CustomGdalEncoder();
OSGeo.GDAL.Gdal.GlobalStringEncoder = encoder;
OSGeo.OGR.Ogr.GlobalStringEncoder = encoder;
OSGeo.OSR.Osr.GlobalStringEncoder = encoder;

@runette

What are related issues/pull requests?

#3825
#14283

Tasklist

• Make sure code is correctly formatted
• Review
• Adjust for comments

Environment

Provide environment details, if relevant:
Mono and dotnet

[runette]

It is probably going to be next week before I can look at this

[runette]

@Mbucari can you rebase against master and run the tests again

[Mbucari]

@runette I rebased to master and incorporated this PR's changes into the new exception helper (#14474).

[runette]

@runette I rebased to master and incorporated this PR's changes into the new exception helper (#14474).

I was about to ask :)

[Mbucari]

This new typedef for char **ignorechange is nice because it handles the extra level of indirection in the in definition below, so cstype doesn't even need to be a ref string. This raises the question, how loathed is the GDAL community to change/remove a function after it's already been made public? If we changed this typedef to just accept a string, then only 2 methods would change: SpatialReference.ImportFromWkt(ref string); and Ogr.CreateGeometryFromWkt(ref string, SpatialReference). This would be more convenient, but it would break things downstream (albeit things which are easily fixed).

This question will be relevant to future additions I have planned, such as adding multi-argument typemaps for arrays (like already exist for Java) which will eliminate the need to pass both the array and the array length.