Initialize a byte array to a certain value, other than the default null? [duplicate]

For small arrays use array initialisation syntax:

var sevenItems = new byte[] { 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20 };

For larger arrays use a standard for loop. This is the most readable and efficient way to do it:

var sevenThousandItems = new byte[7000];
for (int i = 0; i < sevenThousandItems.Length; i++)
{
    sevenThousandItems[i] = 0x20;
}

Of course, if you need to do this a lot then you could create a helper method to help keep your code concise:

byte[] sevenItems = CreateSpecialByteArray(7);
byte[] sevenThousandItems = CreateSpecialByteArray(7000);

// ...

public static byte[] CreateSpecialByteArray(int length)
{
    var arr = new byte[length];
    for (int i = 0; i < arr.Length; i++)
    {
        arr[i] = 0x20;
    }
    return arr;
}