Question

This algorithm is about to store strings from Array A to Array B by storing “A”, “B” to Index 8 and Index 9
I really initiate to make the array size of B to be 10 because later I will put some other things there.

My partial code:

string[] A = new string[]{"A","B"}
string[] B = new string[10]; 
int count;

for(count = 0; count < A.length; count++)
{
      B[count] = A[count]
}

Answer 1

So you want to increment every index with 2:

string[] A = new string[] { "A", "B", "C", "D" };
string[] B = new string[A.Length + 2];
for (int i = 0; i < A.Length; i++)
{
    B[i + 2] = A[i];
}

Demo

Index: 0 Value: 
Index: 1 Value: 
Index: 2 Value: A
Index: 3 Value: B
Index: 4 Value: C
Index: 5 Value: D

Edit: So you want to start with index 0 in B and always leave a gap?

string[] A = new string[] { "A", "B", "C", "D" };
string[] B = new string[A.Length * 2 + 2]; // you wanted to add something other as well
for (int i = 0; i/2 < A.Length; i+=2)
{
    B[i] = A[i / 2];
}

Demo

Index: 0 Value: A
Index: 1 Value: 
Index: 2 Value: B
Index: 3 Value: 
Index: 4 Value: C
Index: 5 Value: 
Index: 6 Value: D
Index: 7 Value: 
Index: 8 Value: 
Index: 9 Value:

Update ” Is there any alternative coding aside from this?”

You can use Linq, although it would be less readable and efficient than a simple loop:

String[] Bs = Enumerable.Range(0, A.Length * 2 + 2) // since you want two empty places at the end
 .Select((s, i) => i % 2 == 0 && i / 2 < A.Length ? A[i / 2] : null)
 .ToArray();

Final Update according to your last comment(start with index 1 in B):

for (int i = 1; (i-1) / 2 < A.Length; i += 2)
{
    B[i] = A[(i-1) / 2];
}

Demo

Index: 0 Value: 
Index: 1 Value: A
Index: 2 Value: 
Index: 3 Value: B
Index: 4 Value: 
Index: 5 Value: C
Index: 6 Value: 
Index: 7 Value: D
Index: 8 Value: 
Index: 9 Value