配列コピーするのに↓だとできるのに
static string[,] ArrayCopy(string[,] array1, int sourceIndex, int destinationIndex, int length)
{
string[,] array2 = new string[array1.GetLength(0), array1.GetLength(1)];

for (int a = 0; a < length; a++)
{
for (int b = 0; b < array1.GetLength(1); b++)
{
array2[a + destinationIndex, b] = array1[a + sourceIndex,b];
}
}
return array2;
}

↓だとできないのはなぜか教えろ
static string[,] ArrayCopy(string[,] array1, int sourceIndex, int destinationIndex, int length,string[,]array2)
{
for (int a = 0; a < length; a++)
{
for (int b = 0; b < array1.GetLength(1); b++)
{
array2[a + destinationIndex, b] = array1[a + sourceIndex,b];
}
}
return array2;
}