Here is problem.

long sum = 0;
Parallel.For(1, 10000, y => { sum1 += y;} );

Solution is ..

Parallel.For<int>(0, result.Count, () => 0, (i, loop, subtotal) =>
{
    subtotal += result[i];
    return subtotal;
},
(x) => Interlocked.Add(ref sum, x)
);

if there are two parameters in this code. For example

long sum1 = 0; long sum2 = 0;
Parallel.For(1, 10000, y => { sum1 += y; sum2=sum1*y; } );

what will we do ? i am guessing that have to use array !

int[] s=new int[2];
Parallel.For<int[]>(0, result.Count, () => s, (i, loop, subtotal) =>
{
    subtotal[0] += result[i];
    subtotal[1] -= result[i];
    return subtotal;
},
(x) => Interlocked.Add(ref sum1, x[0])
//but how about sum1 i tried several way but it doesn't work. 
//for example like that
//(x[0])=> Interlocked.Add (ref sum1, x[0])
//(x[1])=> Interlocked.Add (ref sum2, x[1]));