Adding notes on passing arrays around.

Author: aubertc

source/code/projects/2DarrayExercise/2DarrayExercise/Program.cs

@@ -18,13 +18,13 @@ static void Main()
     Console.WriteLine("We now rotate the array.");
     Rotate(ref matrix1);
     Display(matrix1);
-    
+
     // Example array2
     int[,] matrix2 =
     {
       { 1, 2, 3, 4 },
       { 5, 6, 7, 8 },
-      { 9, 10, 11, 12 }
+      { 9, 10, 11, 12 },
     };
     // End of example array2
     Console.WriteLine("Example 2:");
@@ -64,9 +64,12 @@ static void Rotate(ref int[,] matP)
       {
         tmp[row, col] = matP[
           // This would be incorrect:
-          // tmp.GetLength(0) - col, row 
+          // tmp.GetLength(0) - col, row
           // can you figure out why?
-          tmp.GetLength(1) - col - 1, row
+          tmp.GetLength(1)
+            - col
+            - 1,
+          row
         ];
       }
     }

source/code/projects/ReferenceAndProperties/ReferenceAndProperties/Demo.cs

@@ -0,0 +1,22 @@
+using System;
+
+class Demo
+{
+  private int[] values;
+  public int[] Values
+  {
+    get { return values; }
+  }
+
+  private int value;
+  public int Value
+  {
+    get { return value; }
+  }
+
+  public Demo(int[] valuesP, int valueP)
+  {
+    values = valuesP;
+    value = valueP;
+  }
+}

source/code/projects/ReferenceAndProperties/ReferenceAndProperties/Program.cs

@@ -0,0 +1,32 @@
+using System;
+
+class Program
+{
+  static void Main(string[] args)
+  {
+    int[] values = { 1, 2, 3 };
+    int value = 0;
+    Demo test = new Demo(values, value);
+
+    Console.WriteLine(test.Value);
+    for (int i = 0; i < test.Values.Length; i++)
+    {
+      Console.WriteLine(test.Values[i]);
+    }
+
+    // test.Value = 10;
+    // error CS0200: Property or indexer 'Demo.Value' cannot be assigned to -- it is read only
+    Console.WriteLine(test.Value);
+    // still displays "0"
+    test.Values[0] = 10;
+    // This works!
+    for (int i = 0; i < test.Values.Length; i++)
+    {
+      Console.WriteLine(test.Values[i]);
+    }
+    // Now displays
+    // 10
+    // 2
+    // 3
+  }
+}

source/lectures/misc/references.md

@@ -276,3 +276,29 @@ static void Dummy(out int a)
 ```
 
 would not compile, as C# would give back a message "The out parameter 'a' must be assigned to before controls leaves the current method": an argument passed using the keyword `out` **must** be initialized in the body of the method.
+
+## Security Concerns
+
+Note that reference types requires greater care.
+
+Consider the following `Demo` class implementation:
+
+
+```{download="./code/projects/ReferenceAndProperties.zip"}
+!include code/projects/ReferenceAndProperties/ReferenceAndProperties/Demo.cs
+```
+
+One may think that `values` is "read-only" and can only be given a value when a new object is created, since the `Values` property does not implement a `set` method.
+However, **this is wrong**, as we show now:
+
+```{download="./code/projects/ReferenceAndProperties.zip"}
+!include code/projects/ReferenceAndProperties/ReferenceAndProperties/Program.cs
+```
+
+The following would compile and execute properly, **allowing to change the values in the `Values` array since *its reference* was passed to the `Main` method**.
+
+To avoid this issue, one would have to return a *copy* of the array, for example by copying the array using a `for` loop and returning the copy, or by using the `Clone` method, as follows:
+
+```
+get { return (int[])values.Clone(); }
+```