"Bloke %s drank %5.2f litres of booze and ate %d bananas"
	
"Bloke {0} drank {1,5:f2} litres of booze and ate {2} bananas"
	
#include <string.h>

void convertCtoCSharpFormat(char *dst, const char *src) {
  int k1 = 0, k2 = 0;
  while (*src) {
    while (*src && (*src != '%')) *dst++ = *src++;
    if (*src == '%') {
      const char *percent;
      src++;
      if (*src == '%') { *dst++ = '%'; continue; }
      if (*src == 0) { /* error: unmatched % */; *dst = 0; return; }
      percent = src;
      /* ignore everything between the % and the conversion specifier */
      while (!strchr("diouxXeEfFgGaAcpsn", *src)) src++;

      /* replace with {k} */
      *dst++ = '{';
      if (k2) *dst++ = k2 + '0';
      *dst++ = k1++ + '0';
      if (k1 == 10) { k2++; k1 = 0; }
      /* *src has the conversion specifier if needed */
      /* percent points to the initial character of the conversion directive */
      if (*src == 'f') {
        *dst++ = ',';
        while (*percent != 'f') *dst++ = *percent++;
      }
      *dst++ = '}';
      src++;
    }
  }
  *dst = 0;
}

#ifdef TEST
#include <stdio.h>
int main(void) {
  char test[] = "Bloke %s drank %5.2f litres of booze and ate %d bananas";
  char out[1000];

  convertCtoCSharpFormat(out, test);
  printf("C fprintf string: %snC# format string: %sn", test, out);
  return 0;
}
#endif
	
StringBuilder cString = new StringBuilder("Bloke %s drank %5.2f litres of booze and ate %d bananas");
cString.Replace("%s","{0}");
cString.Replace("%5.2f", "1,5:f2"); // I am unsure of this format specifier
cString.Replace("%d", "{2}");

string newString = String.Format(cString.ToString(), var1, var2, var3);