c#字符串数组的拼接和连接问题

string[] str1 = new string[size1];
string[] str2 = new string[size2];

string[] Str = new string[size1+size2];

Str=str1+str2;（报错）
如何将Str中存储str1与str2

/// <summary>
        /// 将数组a,b进行合并
        /// </summary>
        /// <param name="a"></param>
        /// <param name="b"></param>
        /// <param name="c"></param>
        private static byte[] Combine2(string[] a, string[] b)
        {
            byte[] c = new byte[a.Length + b.Length];

            Array.Copy(a, 0, c, 0, a.Length);
            Array.Copy(b, 0, c, a.Length, b.Length);

            return c;
        }

Str=str1.Concat(str2).ToArray();