dot net qr code library Parameters in Visual Studio .NET

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9 Parameters
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public static void Swap(ref Object a, ref Object b) { Object t = b; b = a; a = t; }
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To swap references to two String objects, you d probably think that you could write code like this:
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public static void SomeMethod() { String s1 = "Jeffrey"; String s2 = "Richter"; Swap(ref s1, ref s2); Console.WriteLine(s1); Console.WriteLine(s2); }
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// Displays "Richter" // Displays "Jeffrey"
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However, this code won t compile . The problem is that variables passed by reference to a method must be of the same type as declared in the method signature . In other words, Swap expects two Object references, not two String references . To swap the two String references, you must do this:
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public static void SomeMethod() { String s1 = "Jeffrey"; String s2 = "Richter"; // Variables that are passed by reference // must match what the method expects. Object o1 = s1, o2 = s2; Swap(ref o1, ref o2); // Now cast the objects back to strings. s1 = (String) o1; s2 = (String) o2; Console.WriteLine(s1); Console.WriteLine(s2); } // Displays "Richter" // Displays "Jeffrey"
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This version of SomeMethod does compile and execute as expected . The reason why the parameters passed must match the parameters expected by the method is to ensure that type safety is preserved . The following code, which thankfully won t compile, shows how type safety could be compromised .
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internal sealed class SomeType { public Int32 m_val; } public sealed class Program { public static void Main() { SomeType st;
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Part II Designing Types
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// The following line generates error CS1503: Argument '1': // cannot convert from 'ref SomeType' to 'ref object'. GetAnObject(out st); Console.WriteLine(st.m_val); } private static void GetAnObject(out Object o) { o = new String('X', 100); } }
In this code, Main clearly expects GetAnObject to return a SomeType object . However, because GetAnObject s signature indicates a reference to an Object, GetAnObject is free to initialize o to an object of any type . In this example, when GetAnObject returned to Main, st would refer to a String, which is clearly not a SomeType object, and the call to Console.WriteLine would certainly fail . Fortunately, the C# compiler won t compile the preceding code because st is a reference to SomeType, but GetAnObject requires a reference to an Object . You can use generics to fix these methods so that they work as you d expect . Here is how to fix the Swap method shown earlier:
public T t b = a = } static void Swap<T>(ref T a, ref T b) { = b; a; t;
And now, with Swap rewritten as above, the following code (identical to that shown before) will compile and run perfectly:
public static void SomeMethod() { String s1 = "Jeffrey"; String s2 = "Richter"; Swap(ref s1, ref s2); Console.WriteLine(s1); Console.WriteLine(s2); }
// Displays "Richter" // Displays "Jeffrey"
For some other examples that use generics to solve this problem, see System.Threading s Interlocked class with its CompareExchange and Exchange methods .
9 Parameters
Passing a Variable Number of Arguments to a Method
It s sometimes convenient for the developer to define a method that can accept a variable number of arguments . For example, the System.String type offers methods allowing an arbitrary number of strings to be concatenated together and methods allowing the caller to specify a set of strings that are to be formatted together . To declare a method that accepts a variable number of arguments, you declare the method as follows:
static Int32 Add(params Int32[] values) { // NOTE: it is possible to pass the 'values' // array to other methods if you want to. Int32 sum = 0; if (values != null) { for (Int32 x = 0; x < values.Length; x++) sum += values[x]; } return sum; }
Everything in this method should look very familiar to you except for the params keyword that is applied to the last parameter of the method signature . Ignoring the params keyword for the moment, it s obvious that this method accepts an array of Int32 values and iterates over the array, adding up all of the values . The resulting sum is returned to the caller . Obviously, code can call this method as follows:
public static void Main() { // Displays "15" Console.WriteLine(Add(new Int32[] { 1, 2, 3, 4, 5 } )); }
It s clear that the array can easily be initialized with an arbitrary number of elements and then passed off to Add for processing . Although the preceding code would compile and work correctly, it is a little ugly . As developers, we would certainly prefer to have written the call to Add as follows:
public static void Main() { // Displays "15" Console.WriteLine(Add(1, 2, 3, 4, 5)); }
You ll be happy to know that we can do this because of the params keyword . The params keyword tells the compiler to apply an instance of the System.ParamArrayAttribute custom attribute to the parameter .
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