CMD, C# & Java won't let me use environment variables set on the same line

In a batch file, you can prompt for values to environment variables using the format of set /p TestVar=Give me a string to save! and retrieve it using %TestVar%. However, when I try to do this in Java or C# and trying to immediately use it does not work:

cmd.exe /K "set /p TestVar=Give me a string to save! && ECHO %TestVar%"

the above code also only ECHOs the literal string "%TestVar%" rather than the string that was set to the TestVar variable.

I want to use that to set environment variables in Java and C# (and I don't want to use built in stuff like https://stackoverflow.com/questions/185208/how-do-i-get-and-set-environment-variables-in-c)

For a Java example,

public class visibleCMD {
	public static void main(String[] args) {
		try {
			Runtime.getRuntime().exec("cmd /c start cmd.exe /K \"set /p TestVar=Give me a string to save! && ECHO %TestVar%\"");
		} catch (Exception e) {
			System.out.println("error");
		}
	}
}

the above code only ECHOs %TestVar%, rather than the string that was set to TestVar, almost as if it just isn't being parsed as a variable at all.

For a C# example,

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace VisibleCMD
{
    class Program
    {
        static void Main(string[] args)
        { 
            System.Diagnostics.Process.Start("CMD.exe", 
                "/K set /p TestVar=Give me a string to save! && ECHO %TestVar%");
        }
    }
}

The problem is with your command line, not with the language you're using. You have to enable delayed expansion if you want to access the variable's value on the same line as it's set. Passing the /V switch will enable delayed expansion of the variable, and using ! instead of % will get the current value of the variable:

Process.Start("CMD.exe", 
    "/V /K set /p TestVar=Give me a string to save! && ECHO !TestVar!");