I have searched and tried so many combinations to implement ToUpper into my program but it simply doesn't work so I'm resorting to the message boards!

I'm coding in C# and I use Visual 6 C++ at Uni and Codeblocks at home and it works on neither.

Currently I'm using (repeatTest=='Y' || repeatTest=='y') but I want to implement <ctype.h> and ToUpper so that even if a lowercase y is entered it still accepts it.

I've tried

char repeatTest = repeatTest.ToUpper();
repeatTest = repeatTest.ToUpper();

both say 'ToUpper' in something not a structure or union

and many other combinations.

Here's my working code

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main()
	char repeatTest='Y';

	while (repeatTest=='Y' || repeatTest=='y')
		char answer=0, correctAnswer=0, questionNumber=1, testResult=0, NumberOfQuestions=1;
		srand (time(0));

                printf ("Test will begin, good luck\n\n\n\n");

                for (NumberOfQuestions=0; NumberOfQuestions!=10; NumberOfQuestions++) {
                        int randomValue1=rand()%12+1;
                        int randomValue2=rand()%12+1;
                        printf("%i. What is %i x %i?    ",questionNumber, randomValue1, randomValue2);
                        scanf("%i", &answer);

                        if (answer==(randomValue1*randomValue2)){
                        printf ("Correct\n\n");

			else {
				printf ("Incorrect\n\n");

               testResult = correctAnswer * 10;
               printf ("You answered %i%% of the questions correctly\n\n",testResult);
               printf ("Would you like to have another go? Y or N\n\n");
               scanf (" %c", &repeatTest);

                if (repeatTest=='N' || repeatTest=='n'){
                        return 0;


Any responses would be appreacted