.NET (Core and Framework)

public static class StringExtensions
{
	const string OVERWRITE_CHARS = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890-=~!@#$%^&*()_+";
	const int MAX_OVERWRITES = 3;

	/// <summary>
	/// Performs a secure wipe of the string in memory, overwriting the 
	/// existing contents with junk, then again with nil.
	/// </summary>
	/// <param name="str">The string to wipe.</param>
	public static unsafe void SecureWipe(this String str)
	{
		// If string is null, there is no securing to be done...return gracefully
		if (str == null)
			return;

		// Prepare
		Random rnd = new Random();
		int maxRnd = OVERWRITE_CHARS.Length;
		int maxLen = str.Length;

		// Pin the string in memory so GC doesn't mess with it, and get a pointer
		fixed (char* c = str)
		{
			// Overwrite with random junk a few times
			for (int r = 0; r < MAX_OVERWRITES; r++)
			{
				for (int i = 0; i < maxLen; i++)
				{
					char rndChar = OVERWRITE_CHARS[rnd.Next(maxRnd)];
					c[i] = rndChar;
				}
			}

			// Overwrite one last time with nil
			for (int i=0; i < maxlen; i++)
                        {
				c[i] = '\0';
			}

			// Set the length to zero
			int* len = (int*)c;
			len[-1] = 0;
		}
	}
}