何となく作ってみました。

Foo.cs

 class Foo
{
    /// <summary>
    ///   Splits bytes up to chunk of bytes which represents a `character`.
    /// Which throws Exception when invalid UTF-8 byte array is given.
    /// </summary>
    public static System.Collections.Generic.List<byte[]> splitCharsFromUTF8(byte[] utf8)
    {
        var ret = new System.Collections.Generic.List<byte[]>();
        for(int i = 0; i < utf8.Length; i++)
        {
            if((utf8[i] & 0x80) == 0)
            {
                ret.Add(new byte[]{utf8[i]});
            }
            else if(i <= utf8.Length - 2 &&
                    ((utf8[i] & 0xE0) == 0xC0) &&
                    ((utf8[i] & 0x1F) >= 0x02) &&
                    ((utf8[i+1] & 0xC0) == 0x80))
            {
                ret.Add(new byte[]{utf8[i], utf8[i+1]});
                i++;
            }
            else if(i <= utf8.Length - 3 &&
                    ((utf8[i] & 0xF0) == 0xE0) &&
                    ((utf8[i+1] & 0xC0) == 0x80) &&
                    ((utf8[i+1] & 0xBF) >= 0x20) &&
                    ((utf8[i+2] & 0xC0) == 0x80))
            {
                ret.Add(new byte[]{utf8[i], utf8[i+1], utf8[i+2]});
                i += 2;
            }
            else if(i <= utf8.Length - 4 &&
                    ((utf8[i] & 0xF8) == 0xF0) &&
                    ((utf8[i+1] & 0xC0) == 0x80) &&
                    ((utf8[i+1] & 0x3F) >= 0x10) &&
                    ((utf8[i+2] & 0xC0) == 0x80) &&
                    ((utf8[i+3] & 0xC0) == 0x80))
            {
                ret.Add(new byte[]{utf8[i], utf8[i+1], utf8[i+2], utf8[i+3]});
                i += 3;
            }
            else
            {
                throw new System.Exception("Invalid UTF8 sequence found at(" + i + ") with value of" + utf8[i]);
            }
        }
        return ret;
    }
}