is java byte the same as C# byte?

Java lacks the unsigned types. In particular, Java lacks a primitive type for an unsigned byte. The Java byte type is signed, while the C# byte is unsigned and sbyte is signed.

Is that because of byte[] in java and c# are different things?

Yes.

• Endianness: Java stores things internally as Big Endian, while .NET is Little Endian by default.
• Signedness: C# bytes are unsigned. Java bytes are signed.

See different results when converting int to byte array - .NET vs Java.

What's the signature of the native function? How do you declare it in Java and in C#?

The most common reason for EntryPointNotFoundException is that function name is mangled (esp. true if function is written in C++) or misspelled.

Another source of problem is 'W' and 'A' suffixes for WinAPI function used to distinguish ANSI and Unicode versions of functions. .NET interop mechanism can try to guess the function suffix, so that may be the source of confusion,