C# 0 The Complete Reference

FailSoftArray

, a variable called 

ErrFlag

 contains the 

outcome of each operation. This field can be examined after each operation to assess the 

success or failure of the operation. (In Chapter 13, you will see a better way to handle errors 

by using C#’s exception subsystem, but for now, using an error flag is an acceptable 

approach.)

The

set

 accessor is shown here. It too prevents a boundary error.

set {

  if(ok(index)) {

    a[index] = value;

    ErrFlag = false;

  }

  else ErrFlag = true;

}

Here, if 

index

 is within bounds, the value passed in 

value

 is assigned to the corresponding 

element. Otherwise, 

ErrFlag

 is set to 

true

. Recall that in an accessor method, 

value

 is an 

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implicit parameter that contains the value being assigned. You do not need to (nor can you) 

declare it.

It is not necessary for an indexer to support both 

get

 and 

set

. You can create a read-only 

indexer by implementing only the 

get

 accessor. You can create a write-only indexer by 

implementing only 

set

.

Indexers Can Be Overloaded

An indexer can be overloaded. The version executed will be the one that has the closest 

type-match between its parameter and the argument used as an index. Here is an example 

that overloads the 

FailSoftArray

 indexer for indexes of type 

double

. The 

double

 indexer 

rounds its index to the nearest integer value.

// Overload the FailSoftArray indexer.

using System;

class FailSoftArray {

  int[] a;    // reference to underlying array

  public int Length; // Length is public

  public bool ErrFlag; // indicates outcome of last operation

  // Construct array given its size.

  public FailSoftArray(int size) {

    a = new int[size];

    Length = size;

  }

  // This is the int indexer for FailSoftArray.

  public int this[int index] {

    // This is the get accessor.

    get {

      if(ok(index)) {

        ErrFlag = false;

        return a[index];

      } else {

        ErrFlag = true;

        return 0;

      }

    }

    // This is the set accessor.

    set {

      if(ok(index)) {

        a[index] = value;

        ErrFlag = false;

      }

      else ErrFlag = true;

    }

  }

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  /* This is another indexer for FailSoftArray.

     This index takes a double argument. It then

     rounds that argument to the nearest integer index. */

  public int this[double idx] {

    // This is the get accessor.

    get {

      int index;

      // Round to nearest int.

      if( (idx - (int) idx) < 0.5) index = (int) idx;

      else index = (int) idx + 1;

      if(ok(index)) {

        ErrFlag = false;

        return a[index];

      } else {

        ErrFlag = true;

        return 0;

      }

    }

    // This is the set accessor.

    set {

      int index;

      // Round to nearest int.

      if( (idx - (int) idx) < 0.5) index = (int) idx;

      else index = (int) idx + 1;

      if(ok(index)) {

        a[index] = value;

        ErrFlag = false;

      }

      else ErrFlag = true;

    }

  }

  // Return true if index is within bounds.

  private bool ok(int index) {

   if(index >= 0 & index < Length) return true;

   return false;

  }

}

// Demonstrate the fail-soft array.

class FSDemo {

  static void Main() {

    FailSoftArray fs = new FailSoftArray(5);

    // Put some values in fs.

    for(int i=0; i < fs.Length; i++)

      fs[i] = i;

    // Now index with ints and doubles.

    Console.WriteLine("fs[1]: " + fs[1]);

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