Binary search a byte array

You should probably use a binary reader to convert your binary data to proper .net types. Since c# is a managed language, and the memory representation of types may depend on the runtime, reading and writing data is generally more involved than just dumping some part of memory to disk.

You mention "fixed-length string", I would assume you mean it is ascii encoded. C# string are UTF16, so you will probably need to convert it to allow it to be used in any meaningful way. See also bitconverter for an alternative way to read integer data.

Once you have converted the data to proper c# types it should be fairly trivial to do a binary search, just put the key and value in separate arrays and use Array.BinarySearch. Or use a custom type that implements IComparable<T>.

Or put the data into some other data structure, like a dictionary, for fast searching.

We can modify standard binary-search algorithms to do this.

We need to modify it to refer to the array in Spans of the width you want. So we multiply the index by that width.

delegate int BytesComparer(ReadOnlySpan<byte> a, ReadOnlySpan<byte> b);

int BinarySearchRecord(byte[] array, int recordWidth, ReadOnlySpan<byte> value, BytesComparer comparer)
{
    return BinarySearchRecord(array, 0, array.Length / recordWidth, value, recordWidth, comparer);
}

int BinarySearchRecord(byte[] array, int startRecord, int count, int recordWidth, ReadOnlySpan<byte> value, BytesComparer comparer)
{
    int lo = startRecord;
    int hi = startRecord + count - 1;
    while (lo <= hi)
    {
        int i = lo + ((hi - lo) >> 1);
        int order = comparer(new ReadOnlySpan<byte>(array, i * recordWidth, recordWidth), value);
 
        if (order == 0) return i;
        if (order < 0)
            lo = i + 1;
        else
            hi = i - 1;
    }
 
    return ~lo;
}

You can use it like this (it requires some bit-twiddling on little-endian systems, in order to sort strings correctly)

var yourArray = Encoding.ASCII.GetBytes("ABLE  xxxxBAKER xxxxCARL  xxxxDELTA xxxxEDGAR xxxxGEORGExxxx");

var valToFind = Encoding.ASCII.GetBytes("DELTA ").Concat(new byte[]{0x1, 0x2, 0x3, 0x4}).ToArray();

var foundIndex = BinarySearchRecord(yourArray, valToFind.Length, new ReadOnlySpan<byte>(valToFind), (a, b) =>
    {
        return BinaryPrimitives.ReverseEndianness(BitConverter.ToInt64(a) & 0xffffffffffff)
            .CompareTo(
               BinaryPrimitives.ReverseEndianness(BitConverter.ToInt64(b) & 0xffffffffffff));
    });

dotnetfiddle