string.Copy deprecation removes a necessary use case with TextRenderer.MeasureText

• .NET Core Version: 3.0 Preview 7
• Have you experienced this same bug with .NET Framework?: No

Problem description: string.Copy was deprecated in .NET Core 3.0 (for good reason), however, I had been using it with TextRenderer.MeasureText, which modifies the string when calling it if the string is too long for the width specified. Without string.Copy, I’m not sure how else you would use this API without string.Copy for now. With the warning showing that this API was removed as strings may be deduplicated in the future, I wasn’t sure about how to properly change this code to be more future-proof, as I’m not sure it can be in its current state.

Minimal repro:

using System;
using System.Drawing;
using System.Windows.Forms;

namespace ConsoleApp10
{
    class Program
    {
        static void Main(string[] args)
        {
            string original = @"C:\New Folder\New Folder\New Folder\New Folder\New Folder\New Folder\text.txt";
            string result = GetCompactedString(original, 4);
            //Result is now correctly "...\text.txt"
            //Expectation is that original is still the very long file path, but it is the same as result
        }

        public static string GetCompactedString(string stringToCompact, int maxWidth)
        {
            //This line is necessary to not cause the bug shown below
            //string copy= string.Copy(stringToCompact);
            string copy = stringToCompact;

            var opts = TextFormatFlags.PathEllipsis | TextFormatFlags.ModifyString;
            TextRenderer.MeasureText(copy, new Font("Segoe UI", 9), new Size(maxWidth, 0), opts);

            // https://connect.microsoft.com/VisualStudio/feedback/details/620330
            int nullByteLoc = copy.IndexOf('\0');
            if (nullByteLoc != -1)
            {
                copy = copy.Substring(0, nullByteLoc);
            }

            return copy;
        }
    }
}

[JeremyKuhne]

Need to validate we don’t allow corrupting the immutability of strings.

[JeremyKuhne]

#3710 tracks replacing the existing TextRenderer API with a safe alternative.