C# Frequently Asked Questions : General

How do I create a constant that is an array?

CSharpFAQ — Fri, 03 Dec 2004 20:33:00 GMT

Strictly speaking you can't, since const can only be applied to a field or local whose value is known at compile time.

In both the lines below, the right-hand is not a constant expression (not in C#).

const int [] constIntArray = newint [] {2, 3, 4};
 // error CS0133: The expression being assigned to 'constIntArray' must be constant
const int [] constIntArrayAnother = {2, 3, 4};
 // error CS0623: Array initializers can only be used in a variable or field
 //               initializer. Try using a new expression instead.

However, there are some workarounds, depending on what it is you want to achieve.

If want a proper .NET array (System.Array) that cannot be reassigned, then static readonly will do for you.

static readonly int [] constIntArray = new int[] {1, 2, 3};

The constIntArray field will be initialized before it its first use.

If, on the other hand, you really need a const set of values (say as an argument to an attribute constructor), then - if you can limit yourself to integral types - an enum would serve you well.

For example:

[Flags]
public enum Role
{
	Administrator = 1,
	BackupOperator = 2,
	// etc. 
}

public class RoleAttribute : Attribute
{
	public RoleAttribute()
	{
		CreateRole = DefaultRole;
	}

	public RoleAttribute(Role role)
	{
		CreateRole = role;
	}

	public Role CreateRole
	{
		get { return this.createRole; }
		set { this.createRole = value; }
	}

	private Role createRole = 0;
	public const Role DefaultRole = Role.Administrator
	 | Role.BackupOperator;
}

[RoleAttribute(RoleAttribute.DefaultRole)]
public class DatabaseAccount
{
	//.............. 
}

RoleAttribute, instead of taking an array, would only take a single argument of flags (appropriately or-ed). If the underlying type of the Role enum is long or ulong, that gives you 64 different Roles.

[Author: SantoshZ]

How do I get and set Environment variables?

CSharpFAQ — Thu, 02 Dec 2004 17:01:00 GMT

Use the System.Environment class.
Specifically the GetEnvironmentVariable and SetEnvironmentVariable methods.

Admitedly, this is not a question specific to C#, but it is one I have seen enough C# programmers ask, and the ability to set environment variables is new to the Whidbey release, as is the EnvironmentVariableTarget enumeration which lets you separately specify process, machine, and user.

Brad Abrams blogged on this way back at the start of this year, and followed up with a solution for pre-Whidbey users.

[Author: SantoshZ]

Is it possible to output the command-line used to build a project in Visual Studio?

CSharpFAQ — Wed, 01 Dec 2004 01:54:00 GMT

Now that Whidbey has been out in Beta for more than a few months, it seems worth revisiting some frequently asked questions which have different (better?) answers now.

In Everett (v7.1) the answer used to be No.

However, in Whidbey (v8.0), the answer is Yes (and No).

For the yes part of the answer, after building, go to the Output Window, select "Show Output from: Build", and about half way down you will see a section like this:


Target "Compile" in project "ConsoleApplication1.csproj" 
Task "Csc" 
Csc.exe /noconfig /warn:4 /define:DEBUG;TRACE /debug+ /optimize- /out:obj\Debug\ConsoleApplication1.exe /reference:C:\WINDOWS\Microsoft.NET\Framework\v2.0.31125\System.Data.dll, C:\WINDOWS.0\Microsoft.NET\Framework\v2.0.31125\System.XML.dll, C:\WINDOWS\Microsoft.NET\Framework\v2.0.31125\System.dll /target:exe /win32icon:App.ico AssemblyInfo.cs Class1.cs 
The task is invoking the IDE's in-process compiler to perform the equivalent of the above command-line.

Now for the no part of the answer. The project system does not actually execute this command line as part of the build process. As the output says, the IDE directly calls its own in-process compiler to perform the equivalent. However, in all cases, you should get the same results using the command line suggested in the output window. If you don't, you could be looking at a bug.

Note: before you cut and paste the build output to the command line, remember to add the path to CSC.EXE

[Author: SantoshZ]

How can I subscribe to events exposed by a remoted object?

CSharpFAQ — Thu, 29 Apr 2004 04:34:00 GMT

Delegates require information about the type that the method is associated with in order to make a call. In a single app-domain, this isn't a problem, because the server (the object firing the events) has access to the type information for the client (which has the event handlers) by way of the delegate.

However, during remoting, the server most likely does not have any information about the client. If you want events from the server to fire in the client app domain, then the client must derive from MarshalByRefObject. This is required so that the server will call back into the client, as opposed to a copy of the client object that is passed to the server.

A simple way to do this is to place a copy of the client assembly in the same directory where the server directory is. While this will work, it is not an elegant solution, as it exposes the client type unecessarily.

A more elegant solution (albiet more complex), is to have a single assembly that is referenced by both the client and the server. This assembly will have a shim class which will expose methods that share the signature of the events that you want to shim. The assembly will also provide an interface with methods that share the signature of the events as well. The shim will then take a reference to the interface, and the methods will aggregate the call to the interface that was passed in (I recommend the constructor as the place to pass the interface implementation).

Finally, the client object will implement the interface. When subscribing to the events that the server exposes, attach the delegates to the methods on the shim, which will be passed your client object's implementation.

It is also important to note that in order for this to work, the TypeFilterLevel property on the sink provider for the server needs to be set to TypeFilterLevel.Full.

Why does my Windows Form project not use visual styles in XP even when I call Application.EnableVisualStyles?

CSharpFAQ — Thu, 29 Apr 2004 04:15:00 GMT

If you set a property on a Windows Forms control which forces the creation of the control (e.g. the SelectedIndex property on the ComboBox class), the control (and perhaps the rest of the form) will not render with visual styles enabled.

The resolution is to place the code that sets these properties in an event handler for the Load event on the form/control.

How can I update my user interface from a thread that did not create it?

CSharpFAQ — Thu, 18 Mar 2004 03:58:00 GMT

When performing any action on a control which requires the updating of a user interface element (e.g. setting the Text property on almost any class derived from Control, updating the data source behind a DataGrid), these operations MUST take place on the thread that created the UI element.

In order to do this, the Control class provides the Invoke method, which will take a delegate and execute it on the thread that the UI element was created on. In order to use this, one must declare a function that performs the UI operation. For example, say a form has a TextBox on it named m_TextBox. To update the text from another thread, create a method that will update the Text property on the TextBox:

// The declaration of the textbox.
private TextBox m_TextBox;

// Updates the textbox text.
private void UpdateText(string text)
{
  // Set the textbox text.
  m_TextBox.Text = text;
}

Now, create a delegate that has the same signature as the method that was previously defined:

public delegate void UpdateTextCallback(string text);

In your thread, you can call the Invoke method on m_TextBox, passing the delegate to call, as well as the parameters.

m_TextBox.Invoke(new UpdateTextCallback(this.UpdateText), 
            new object[]{”Text generated on non-UI thread.”});

Note: Do not create a method that matches the EventHandler delegate signature and pass that. The implementation of Invoke on the Control class will not take into account the parameters passed to Invoke if the type of the delegate is EventHandler. It will pass the control that Invoke was called on for the sender parameter as well as the value returned by EventArgs.Empty for the e parameter.

[Author: Nicholas Paldino]

Where can I find sample C# code for simple threading?

CSharpFAQ — Mon, 15 Mar 2004 23:02:00 GMT

Refer to the System.Threading namespace on MSDN for full details. Meanwhile here is a quick taste.

using System; 
using System.Threading; 

class ThreadTest 
{ 
    public void Runme() 
    { 
        Console.WriteLine("Runme Called"); 
        Thread.Sleep(10000); 
    }

    public static void Main(String[] args)
    { 
        ThreadTest b = new ThreadTest(); 
        Thread t = new Thread(new ThreadStart(b.Runme)); 
        t.Start(); 

        Console.WriteLine("Thread 't' started.");
        Console.WriteLine("There is no telling when " + 
                          "'Runme' will be invoked. "); 

        t.Join(); 

        Console.WriteLine("Thread 't' has ended."); 
    }
}

[Author: Santosh Zachariah]