String Constructor

String constructors fall into two categories: those without pointer parameters, and those with pointer parameters. The constructors that use pointers are not CLS-compliant. In addition, Visual Basic does not support the use of pointers, and C# requires code that uses pointers to run in an unsafe context. For more information, see the unsafe keyword in the C# Reference.

For additional guidance on choosing an overload, see Which method do I call?

String(Char[] value )

Initializes the new instance to the value indicated by an array of Unicode characters. This constructor copies Unicode characters (example).

String(Char[] value , Int32 startIndex , Int32 length )

Initializes the new instance to the value indicated by an array of Unicode characters, a starting character position within that array, and a length (example).

String(Char c , Int32 count )

Initializes the new instance to the value indicated by a specified Unicode character repeated a specified number of times (example).

String(char* value )

(Not CLS-compliant) Initializes the new instance to the value indicated by a pointer to an array of Unicode characters (example).

Permission: SecurityCriticalAttribute, requires full trust for the immediate caller. This member cannot be used by partially trusted or transparent code.

String(char* value , Int32 startIndex , Int32 length )

(Not CLS-compliant) Initializes the new instance to the value indicated by a pointer to an array of Unicode characters, a starting character position within that array, and a length. The constructor copies the Unicode characters from value starting at index startIndex and ending at index startIndex + length – 1 (example).

Permission: SecurityCriticalAttribute, requires full trust for the immediate caller. This member cannot be used by partially trusted or transparent code.

String(SByte* value )

(Not CLS-compliant) Initializes the new instance to the value indicated by a pointer to an array of 8-bit signed integers. The array is assumed to represent a string encoded using the current system code page (that is, the encoding specified by Encoding.Default). The constructor processes characters from value starting from the location specified by the pointer until a null character (0x0000) is reached (example).

Permission: SecurityCriticalAttribute, requires full trust for the immediate caller. This member cannot be used by partially trusted or transparent code.

String(SByte* value , Int32 startIndex , Int32 length )

(Not CLS-compliant) Initializes the new instance to the value indicated by a pointer to an array of 8-bit signed integers, a starting position within that array, and a length. The constructor processes characters from value starting at startIndex and ending at startIndex + length – 1 (example).

Permission: SecurityCriticalAttribute, requires full trust for the immediate caller. This member cannot be used by partially trusted or transparent code.

String(SByte* value , Int32 startIndex , Int32 length , Encoding enc )

(Not CLS-compliant) Initializes the new instance to the value indicated by a pointer to an array of 8-bit signed integers, a starting position within that array, a length, and an Encoding object.

Permissiony: SecurityCriticalAttribute, requires full trust for the immediate caller. This member cannot be used by partially trusted or transparent code.

Here is a complete list of parameters used by String constructors that do not include a pointer parameter. For the parameters used by each overload, see the overload syntax above.

Here is a complete list of parameters used by String constructors that include a pointer parameter. For the parameters used by each overload, see the overload syntax above.

Here's a list of exceptions thrown by constructors that do not include pointer parameters.

Here's a list of exceptions thrown by constructors that include pointer parameters.

The most commonly used technique for creating strings programmatically is simple assignment, as illustrated in this example. The String class also includes four types of constructor overloads that let you create strings from the following values:

• From a character array (an array of UTF-16-encoded characters). You can create a new String object from the characters in the entire array or a portion of it. The String(Char[]) constructor copies all the characters in the array to the new string. The String(Char[], Int32, Int32) constructor copies the characters from index startIndex to index startIndex + length – 1 to the new string. If length is zero, the value of the new string is String.Empty.

  If your code repeatedly instantiates strings that have the same value, you can improve application performance by using an alternate means of creating strings. For more information, see Handling repetitive strings.

• From a single character that is duplicated zero, one, or more times, by using the String(Char, Int32) constructor. If count is zero, the value of the new string is String.Empty.

• From a pointer to a null-terminated character array, by using the String(Char*) or String(Char*, Int32, Int32) constructor. Either the entire array or a specified range can be used to initialize the string. The constructor copies a sequence of Unicode characters starting from the specified pointer or from the specified pointer plus startIndex and continuing to the end of the array or for length characters. If value is a null pointer or length is zero, the constructor creates a string whose value is String.Empty. If the copy operation proceeds to the end of the array and the array is not null-terminated, the constructor behavior is system-dependent. Such a condition might cause an access violation.

  The array must contain Unicode characters. In C++, this means that the character array must be defined either as the managed Char[] type or the unmanaged wchar_t[] type.

  If the String(Char*) overload is called and the array is not null-terminated, or if the String(Char*, Int32, Int32) overload is called and startIndex + length -1 includes a range that it outside the memory allocated for the sequence of characters, the behavior of the constructor is system-dependent, and an access violation may occur. In addition, on the Intel Itanium processor, calls to the String(Char*, Int32, Int32) constructor may throw a DataMisalignedException exception. If this occurs, call the String(Char[], Int32, Int32) instead.

• From a pointer to a signed byte array. Either the entire array or a specified range can be used to initialize the string. The sequence of bytes can be interpreted by using the default code page encoding, or an encoding can be specified in the constructor call. If the constructor tries to instantiate a string from an entire array that is not null-terminated, or if the range of the array from value + startIndex to value + startIndex + length -1 is outside of the memory allocated for the array, the behavior of this constructor is system-dependent, and an access violation may occur.

  The three constructors that include a signed byte array as a parameter are designed primarily to convert a C++ char array to a string, as shown in this example:

  C++

  Copy

  using namespace System;
  
  void main()
  {
        char chars[] = { 'a', 'b', 'c', 'd', '\x00' };
  
        char* charPtr = chars;
        String^ value = gcnew String(charPtr);
  
        Console::WriteLine(value);
  }
  // The example displays the following output: 
  //      abcd
  

  Because the String(SByte*) and String(SByte*, Int32, Int32) constructors interpret value by using the default ANSI code page, calling these constructors with identical byte arrays may create strings that have different values on different systems.

Apps that parse or decode streams of text often use the String(Char[], Int32, Int32) constructor or the StringBuilder.Append(Char[], Int32, Int32) method to convert sequences of characters into a string. Repeatedly creating new strings with the same value instead of creating and reusing one string wastes memory. If you are likely to create the same string value repeatedly by calling the String(Char[], Int32, Int32) constructor, even if you do not know in advance what those identical string values may be, you can use a lookup table instead.

For example, suppose you read and parse a stream of characters from a file that contains XML tags and attributes. When you parse the stream, you repeatedly encounter certain tokens (that is, sequences of characters that have a symbolic meaning). Tokens equivalent to the strings "0", "1", "true", and "false" are likely to occur frequently in an XML stream.

Instead of converting each token into a new string, you can create a System.Xml.NameTable object to hold commonly occurring strings. The NameTable object improves performance, because it retrieves stored strings without allocating temporary memory. When you encounter a token, use the NameTable.Get(Char[], Int32, Int32) method to retrieve the token from the table. If the token exists, the method returns the corresponding string. If the token does not exist, use the NameTable.Add(Char[], Int32, Int32) method to insert the token into the table and to get the corresponding string.

The following example creates a new string by assigning it a string literal. It creates a second string by assigning the value of the first string to it. These are the two most common ways to instantiate a new String object.

using System;

public class Example
{
   public static void Main()
   {
      String value1 = "This is a string.";
      String value2 = value1;
      Console.WriteLine(value1);
      Console.WriteLine(value2);
   }
}
// The example displays the following output: 
//    This is a string. 
//    This is a string.

The following example demonstrates how to create a new String object from a character array.

// Unicode Mathematical operators 
char [] charArr1 = {'\u2200','\u2202','\u200F','\u2205'};
String szMathSymbols = new String(charArr1);

// Unicode Letterlike Symbols 
char [] charArr2 = {'\u2111','\u2118','\u2122','\u2126'};
String szLetterLike = new String (charArr2);

// Compare Strings - the result is false
Console.WriteLine("The Strings are equal? " +
    (String.Compare(szMathSymbols, szLetterLike)==0?"true":"false") );

The following example demonstrates how to create a new String object from a portion of a character array, and how to create a new String object that contains multiple occurrences of a single character.

// Create a Unicode String with 5 Greek Alpha characters
String szGreekAlpha = new String('\u0391',5);
// Create a Unicode String with a Greek Omega character
String szGreekOmega = new String(new char [] {'\u03A9','\u03A9','\u03A9'},2,1);

String szGreekLetters = String.Concat(szGreekOmega, szGreekAlpha, szGreekOmega.Clone());

// Examine the result
Console.WriteLine(szGreekLetters);

// The first index of Alpha 
int ialpha = szGreekLetters.IndexOf('\u0391');
// The last index of Omega 
int iomega = szGreekLetters.LastIndexOf('\u03A9');

Console.WriteLine("The Greek letter Alpha first appears at index " + ialpha +
    " and Omega last appears at index " + iomega + " in this String.");

The following example demonstrates how to create a new String object from a pointer to an array of characters. The C# example must be compiled by using the /unsafe compiler switch.

using System;

public class Example
{
   public static unsafe void Main()
   {
      char[] characters = { 'H', 'e', 'l', 'l', 'o', ' ', 
                            'w', 'o', 'r', 'l', 'd', '!', '\u0000' };
      string value;

      fixed (char* charPtr = characters) {
         value = new String(charPtr);
      }                            
      Console.WriteLine(value);
   }
}
// The example displays the following output: 
//        Hello world!

The following example examines the elements of a character array for either a period or an exclamation point. If one is found, it instantiates a string from the characters in the array that precede the punctuation symbol. If not, it instantiates a string with the entire contents of the array. The C# example must be compiled using the /unsafe compiler switch.

using System;

public class Example
{
   public static unsafe void Main()
   {
      char[] characters = { 'H', 'e', 'l', 'l', 'o', ' ', 
                            'w', 'o', 'r', 'l', 'd', '!', '\u0000' };
      String value;

      fixed (char* charPtr = characters) {
         int length = 0;
         Char* iterator = charPtr;

         while (*iterator != '\x0000')
         {
            if (*iterator == '!' || *iterator == '.')
               break;
            iterator++;
            length++;
         }
         value = new String(charPtr, 0, length);
      }
      Console.WriteLine(value);
   }
}
// The example displays the following output: 
//      Hello World

The following example demonstrates how you can create an instance of the String class with the String(SByte*) constructor.

unsafe
{
    // Null terminated ASCII characters in an sbyte array
    String szAsciiUpper = null;
    sbyte[] sbArr1 = new sbyte[] { 0x41, 0x42, 0x43, 0x00 };
    // Instruct the Garbage Collector not to move the memory 
    fixed(sbyte* pAsciiUpper = sbArr1)
    {
        szAsciiUpper = new String(pAsciiUpper);
    }
    String szAsciiLower = null;
    sbyte[] sbArr2 = { 0x61, 0x62, 0x63, 0x00 };
    // Instruct the Garbage Collector not to move the memory 
    fixed(sbyte* pAsciiLower = sbArr2)
    {
        szAsciiLower = new String(pAsciiLower, 0, sbArr2.Length);
    }
    // Prints "ABC abc"
    Console.WriteLine(szAsciiUpper + " " + szAsciiLower);

    // Compare Strings - the result is true
    Console.WriteLine("The Strings are equal when capitalized ? " +
        (String.Compare(szAsciiUpper.ToUpper(), szAsciiLower.ToUpper())==0?"true":"false") );

    // This is the effective equivalent of another Compare method, which ignores case
    Console.WriteLine("The Strings are equal when capitalized ? " +
        (String.Compare(szAsciiUpper, szAsciiLower, true)==0?"true":"false") );
}

.NET Framework

All overloads are supported in: 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

All overloads are supported in: 4, 3.5 SP1

Portable Class Library

All overloads without an SByte* parameter are supported

.NET for Windows Store apps

All overloads without an SByte* parameter are supported in: Windows 8