testcase.cs:
using System;
using System.Runtime.InteropServices;

public class test{

static void Main(){

double a = 3.5;
float b = 0.2f;
int c = 1;
Console.WriteLine("C#:");
Console.WriteLine("Double: "+a);
Console.WriteLine("Float: "+b);
Console.WriteLine("Int: "+c);
print_values_c(a,b,c);
}
//pinvoke signature
[DllImport("testcase.so")]
static extern void print_values_c (double a, float b,int c);

}

testcase.c:
#include <stdio.h>

void print_values_c (double a, float b, int c){
printf("C:\n");
printf("Double: %g\n",a);
printf("Float: %g\n",b);
printf("Int: %d\n",c);
} 

output:
C#:
Double: 3,5
Float: 0,2
Int: 1
C:
Double: 3,5
Float: 0,2
Int: 1