Exposing a Property: The PrimeGenerator Control Example

Exposing a Property: The PrimeGenerator Control Example

Now that we have seen the main steps for authoring and deploying a server control, we'll write a control that exposes a property and does some useful work. Our next sample, PrimeGenerator , is a simple server control that computes and displays prime numbers less than or equal to a given number. The main task of our control is to render the computed primes to the client.

A user of our control should be able to specify an upper limit for the primes' computation. For this purpose, we will expose a property named Number from our control. Properties are accessed similarly to the way fields are, but they are implemented as accessor methods , effectively acting as "smart" fields. (We described the property construct in Chapter 3, "Component Programming Overview.")

Listing 5-3 provides the complete code for the PrimeGenerator control. PrimeGenerator derives from Control , exposes the Number property, and overrides the Render method to output the result of the computation of primes.

Listing 5-3 PrimeGenerator.cs

 usingSystem; usingSystem.Web.UI; namespaceMSPress.ServerControls{ publicclassPrimeGenerator:Control{ privateint_number; publicintNumber{ get{return_number;} set{_number=value;} } protectedoverridevoidRender(HtmlTextWriterwriter){ //Sieveisaclassthatcomputesprimes. //Wewilldefinethisclasslater. int[]primes=Sieve.GetPrimes(Number); //Noticethatwewritesuccessivestringstothe //outputstreaminsteadofconcatenatingtheminto //longerstringsbeforewriting.Thisismoreefficient //thanstringconcatenation. writer.Write("Primeslessthanorequalto:"); writer.Write(Number); writer.Write("<br>"); for(inti=0;i<primes.Length;i++){ writer.Write(primes[i]); writer.Write(""); } writer.Write("<br>"); } } } 

All that remains to do is implement a utility class that contains the logic for computing primes. The implementation of this class is not important for control authoring; we have included it here for completeness.

Sieve of Eratosthenes

The sieve of Eratosthenes (circa 275 “195 B.C.) is an elegant algorithm for computing primes smaller than a given number. The algorithm proceeds like this: Given a set of positive integers from 2 to N , start with the number 2 and cross out all its multiples . Repeat with each successive number that is not crossed out ”and thus must be a prime because it has no factors smaller than itself. When you are finished, you will be left with only prime numbers less than or equal to N . Notice that when you get to a prime number i, you have to consider only its multiples with integers j >= i. (This is because multiples with integers j < i have already been considered .) The algorithm terminates when you reach a prime greater than the square root of N .

We expose the prime computation functionality from an independent class named Sieve because that functionality does not belong in a control. Listing 5-4 shows the code for the Sieve class.

Listing 5-4 Sieve.cs

 usingSystem; usingSystem.Collections; namespaceMSPress.ServerControls{ //SieveisautilityclassthatusesthesieveofEratosthenes //algorithmtocomputeprimeslessthanorequalto //agivenpositiveinteger. internalsealedclassSieve{ privateSieve(){} publicstaticint[]GetPrimes(intn){ //Useanarrayofn+1bitstocorrespondto //thenumbersfrom0ton.Initially,setallthebits //totrue.Thegoalofthecomputationistocyclethrough //thebitarrayusingthesieveofEratosthenesalgorithm //andtosetthebitscorrespondingtononprimestofalse. //Theremainingtruebitsrepresentprimes. BitArrayflags=newBitArray(n+1,true); for(inti=2;i<=(int)Math.Sqrt(n);i++){ if(flags[i]){ for(intj=i;j*i<=n;j++){ flags[i*j]=false; } 

 } } //Countthenumberofprimes<=n. intcount=0; for(inti=2;i<=n;i++){ if(flags[i])count++; } //Createanintarraytostoretheprimes. int[]primes=newint[count]; //Checkbitflagsandpopulateprimes //arraywithnumberscorrespondingto //truebits. for(inti=2,j=0;j<count;i++){ if(flags[i])primes[j++]=i; } returnprimes; } } } 

Deriving from WebControl

If we derive our custom control from WebControl instead of from Control , our control inherits many additional properties, such as Font , Height , Width , and other style- related properties that enable a page developer to customize the visual appearance of the control. When deriving from WebControl , keep the following stipulations in mind:

• Include a reference to the System.Web.UI.WebControls namespace.

• Do not directly write to the output stream by overriding the Render method because the Render method of WebControl implements logic to render the outer tag with style information. Instead, override RenderContents if you want to provide content to render within the control's tags. We will explain this in more detail in Chapter 8.

• Override the TagKey property or the TagName property (in the case of a nonstandard HTML tag) inherited from WebControl if you want to render a tag that is different from <span> . By default, WebControl renders an HTML <span> tag. In our example, we will not override either of these properties.

Listing 5-5 highlights the changes to our control when it derives from WebControl .

Listing 5-5 StyledPrimeGenerator.cs

 usingSystem; usingSystem.Web.UI;  usingSystem.Web.UI.WebControls;  namespaceMSPress.ServerControls{  publicclassStyledPrimeGenerator:WebControl  { privateint_number; publicintNumber{ get{return_number;} set{_number=value;} }  protectedoverridevoidRenderContents(HtmlTextWriterwriter)  { //NotethattheimplementationoftheRenderContents //methodisidenticaltotheimplementationofRenderin //PrimeGenerator. int[]primes=Sieve.GetPrimes(Number); writer.Write("Primeslessthanorequalto:"); writer.Write(Number); writer.Write("<br>"); for(inti=0;i<primes.Length;i++){ writer.Write(primes[i]); writer.Write(""); } writer.Write("<br>"); } } } 

Test Page for the PrimeGenerator Control

The page shown in Listing 5-6 tests the PrimeGenerator and StyledPrimeGenerator controls. Note that StyledPrimeGenerator inherits properties such as Height , Width , ForeColor , and BackColor for free because it derives from WebControl .

Listing 5-6 PrimeGeneratorTest.aspx

 <%@PageLanguage="C#"%> <%@RegisterTagPrefix="msp"NameSpace="MSPress.ServerControls" Assembly="MSPress.ServerControls"%> <html> <head> <scriptrunat="server"> voidButtonClicked(objectsender,EventArgse){ primegen1.Number=Int32.Parse(num1.Text); primegen2.Number=Int32.Parse(num2.Text); sprimegen1.Number=Int32.Parse(num3.Text); } </script> </head> <body> <formrunat="server"> <br> <br> <msp:PrimeGeneratorNumber="15"id="primegen1"runat="server"/> <br> <msp:PrimeGeneratorNumber="100"id="primegen2"runat="server"/> <br> <msp:StyledPrimeGeneratorNumber="51"id="sprimegen1" Width="500"Font-Name="Verdana"Font-Size="14pt" BackColor="#EEEEEE"ForeColor="Black"runat="server"/> <br> Enter3positiveintegers: <asp:TextBoxid="num1"width="50"text="0"runat="server"/> <asp:TextBoxid="num2"width="50"text="0"runat="server"/> <asp:TextBoxid="num3"width="50"text="0"runat="server"/> <br> <asp:Buttontext="Submit"id="submit" onclick="ButtonClicked"runat="server"/> <br> </form> </body> </html> 

When you access PrimeGeneratorTest.aspx, the output in your browser should be similar to that shown in Figure 5-3. The third output block is rendered by StyledPrimeGenerator , which is visually distinct because of values assigned to some of its style properties.

Figure 5-3. The PrimeGeneratorTest.aspx page that tests the PrimeGenerator and StyledPrimeGenerator controls, viewed in a browser