char[] c = new char[] { 'a','b','c','d'};
string s = new string(c);
 Console.WriteLine("{0}",s);

[ComVisible(true)]
  [Serializable]
  public sealed class String : IComparable, ICloneable, IConvertible, IComparable<string>, IEnumerable<char>, IEnumerable, IEquatable<string>
  {
    public static readonly string Empty = "";
    private const int TrimHead = 0;
    private const int TrimTail = 1;
    private const int TrimBoth = 2;
    private const int charPtrAlignConst = 1;
    private const int alignConst = 3;
    [NonSerialized]
    private int m_stringLength;
    [ForceTokenStabilization]
    [NonSerialized]
    private char m_firstChar;

    internal char FirstChar
    {
      get
      {
        return this.m_firstChar;
      }
    }

    [IndexerName("Chars")]
    public char this[int index] { [SecuritySafeCritical, MethodImpl(MethodImplOptions.InternalCall)] get; }

    public int Length { [SecuritySafeCritical, MethodImpl(MethodImplOptions.InternalCall)] get; }

    static String()
    {
    }

    [CLSCompliant(false)]
    [SecurityCritical]
    [MethodImpl(MethodImplOptions.InternalCall)]
    public String(char* value);

    [SecurityCritical]
    [CLSCompliant(false)]
    [MethodImpl(MethodImplOptions.InternalCall)]
    public String(char* value, int startIndex, int length);

    [CLSCompliant(false)]
    [SecurityCritical]
    [MethodImpl(MethodImplOptions.InternalCall)]
    public String(sbyte* value);

    [CLSCompliant(false)]
    [SecurityCritical]
    [MethodImpl(MethodImplOptions.InternalCall)]
    public String(sbyte* value, int startIndex, int length);

    [SecurityCritical]
    [CLSCompliant(false)]
    [MethodImpl(MethodImplOptions.InternalCall)]
    public String(sbyte* value, int startIndex, int length, Encoding enc);

    [SecuritySafeCritical]
    [MethodImpl(MethodImplOptions.InternalCall)]
    public String(char[] value, int startIndex, int length);

    [SecuritySafeCritical]
    [MethodImpl(MethodImplOptions.InternalCall)]
    public String(char[] value);

    [SecuritySafeCritical]
    [MethodImpl(MethodImplOptions.InternalCall)]
    public String(char c, int count);

------解决方案--------------------
话说还有一种方式可以清晰的知道，.net你只能靠反编译，但是你可以去看.net的linux实现：mono的源代码，应该是类似的