Introduce string builder factory template.

[paulomorgado]

With this PR, the creation of the StringBuilder for building operation URLs is moved to the Client.Class.StringBuilder.liquid template.

This allows the user to replace it with a template that uses, for example, a StringBuilder pool.

[yesmey]

How common is this scenario? I think if you find a GC allocated StringBuilder to effect your performance, you might not want to use a generated REST client in the first place...
If this is approved though, I think the avarage url length is a lot shorter than 8000 characters.

[paulomorgado]

If you run this code:

var chunk = typeof(StringBuilder).GetField("m_ChunkPrevious", BindingFlags.NonPublic | BindingFlags.Instance);
var sb = new StringBuilder();
var previousCapacity = sb.Capacity;
for (var i = 0; i < 10_000; i++)
{
    sb.Append('*');
    if (sb.Capacity != previousCapacity)
    {
        previousCapacity = sb.Capacity;
        var j = 0;
        var s = sb;
        while (s is not null)
        {
            if (j > 0)
            {
                Console.Write(new string(' ', j * 4));
            }
            Console.WriteLine($"Capacity: {s.Capacity}");
            s = (StringBuilder)chunk.GetValue(s);
            j++;
        }
    }
}

you'll see that the StringBuilder, by default, is a linked list of StringBuilder instances with 16 characters.

This is especially painful if you need to traverse the chain. It is especially painful if you are inserting, removing or replacing characters.

PR #4585 removed replace operations and generates append only operations.

By moving this to its own template, it allows users to use any source compatible version they want. Including special implementations (.NET is full of private ValueStringBuilder implementations) and pooling.

The 8000-character initial size is something I anticipate no one reaches and is easily collected by the GC.

[yesmey]

I know how StringBuilder works internally, I'm just saying I highly doubt theres a need out there to want to pool the StringBuilder.

I'm not against using a default capacity to the StringBuilder object though, but I still think 8000 is too large.
The reason is because the IIS maximum url length defaults to 4096, and at this point the HttpClient likely already has a BaseAddress attached to it, so it won't be part of the appending here. So even though 8000 is too low for LOH its still an unnecessarily big allocation.

By the way, how are you supposed to override the implementation of GetStringBuilder() from this class? And private partial static void ReturnStringBuilder is an illegal method declaration, you need to make it static partial void ReturnStringBuilder.

[paulomorgado]

By the way, how are you supposed to override the implementation of GetStringBuilder() from this class? And private partial static void ReturnStringBuilder is an illegal method declaration, you need to make it static partial void ReturnStringBuilder.

Replacing the template for generation.

[yesmey]

I think this part could just be written as new System.Collections.Generic.Dictionary<string, System.Collections.Generic.IEnumerable<string>>(response_.Headers);

[paulomorgado]

This was supposed to have been addressed in #4586 and improved in #4602.