Walkthrough: Creating and Using Dynamic Objects

Domains: C#

Dynamic objects expose members such as properties and methods at run time, instead of in at compile time. This enables you to create objects to work with structures that do not match a static type or format. For example, you can use a dynamic object to reference the HTML Document Object Model (DOM), which can contain any combination of valid HTML markup elements and attributes. Because each HTML document is unique, the members for a particular HTML document are determined at run time. A common method to reference an attribute of an HTML element is to pass the name of the attribute to the GetProperty method of the element. To reference the id attribute of the HTML element <div id="Div1">, you first obtain a reference to the <div> element, and then use divElement.GetProperty("id"). If you use a dynamic object, you can reference the id attribute as divElement.id.

Dynamic objects also provide convenient access to dynamic languages such as IronPython and IronRuby. You can use a dynamic object to refer to a dynamic script that is interpreted at run time.

You reference a dynamic object by using late binding. In C#, you specify the type of a late-bound object as dynamic. In Visual Basic, you specify the type of a late-bound object as Object. For more information, see dynamic and Early and Late Binding.

You can create custom dynamic objects by using the classes in the System.Dynamic namespace. For example, you can create an ExpandoObject and specify the members of that object at run time. You can also create your own type that inherits the DynamicObject class. You can then override the members of the DynamicObject class to provide run-time dynamic functionality.In this walkthrough you will perform the following tasks:

Create a custom object that dynamically exposes the contents of a text file as properties of an object.
Create a project that uses an IronPython library.

Prerequisites

You need IronPython for .NET to complete this walkthrough. Go to their Download page to obtain the latest version.

Note

Your computer might show different names or locations for some of the Visual Studio user interface elements in the following instructions. The Visual Studio edition that you have and the settings that you use determine these elements. For more information, see Personalizing the IDE.

Creating a Custom Dynamic Object

The first project that you create in this walkthrough defines a custom dynamic object that searches the contents of a text file. Text to search for is specified by the name of a dynamic property. For example, if calling code specifies dynamicFile.Sample, the dynamic class returns a generic list of strings that contains all of the lines from the file that begin with "Sample". The search is case-insensitive. The dynamic class also supports two optional arguments. The first argument is a search option enum value that specifies that the dynamic class should search for matches at the start of the line, the end of the line, or anywhere in the line. The second argument specifies that the dynamic class should trim leading and trailing spaces from each line before searching. For example, if calling code specifies dynamicFile.Sample(StringSearchOption.Contains), the dynamic class searches for "Sample" anywhere in a line. If calling code specifies dynamicFile.Sample(StringSearchOption.StartsWith, false), the dynamic class searches for "Sample" at the start of each line, and does not remove leading and trailing spaces. The default behavior of the dynamic class is to search for a match at the start of each line and to remove leading and trailing spaces.

To create a custom dynamic class

Start Visual Studio.
On the File menu, point to New and then click Project.
In the New Project dialog box, in the Project Types pane, make sure that Windows is selected. Select Console Application in the Templates pane. In the Name box, type DynamicSample, and then click OK. The new project is created.
Right-click the DynamicSample project and point to Add, and then click Class. In the Name box, type ReadOnlyFile, and then click OK. A new file is added that contains the ReadOnlyFile class.
At the top of the ReadOnlyFile.cs or ReadOnlyFile.vb file, add the following code to import the System.IO and System.Dynamic namespaces.
```
		using System.IO;
using System.Dynamic;
```
The custom dynamic object uses an enum to determine the search criteria. Before the class statement, add the following enum definition.
```
		public enum StringSearchOption
{
    StartsWith,
    Contains,
    EndsWith
}
```
Update the class statement to inherit the DynamicObject class, as shown in the following code example.
```
		class ReadOnlyFile : DynamicObject
```

Add the following code to the ReadOnlyFile class to define a private field for the file path and a constructor for the ReadOnlyFile class.

		// Store the path to the file and the initial line count value.
private string p_filePath;

// Public constructor. Verify that file exists and store the path in 
// the private variable.
public ReadOnlyFile(string filePath)
{
    if (!File.Exists(filePath))
    {
        throw new Exception("File path does not exist.");
    }

    p_filePath = filePath;
}

Add the following GetPropertyValue method to the ReadOnlyFile class. The GetPropertyValuemethod takes, as input, search criteria and returns the lines from a text file that match that search criteria. The dynamic methods provided by the ReadOnlyFile class call the GetPropertyValuemethod to retrieve their respective results.

		public List<string> GetPropertyValue(string propertyName,
                                     StringSearchOption StringSearchOption = StringSearchOption.StartsWith,
                                     bool trimSpaces = true) 
{
    StreamReader sr = null;
    List<string> results = new List<string>();
    string line = "";
    string testLine = "";

    try
    {
        sr = new StreamReader(p_filePath);

        while (!sr.EndOfStream)
        {
            line = sr.ReadLine();

            // Perform a case-insensitive search by using the specified search options.
            testLine = line.ToUpper();
            if (trimSpaces) { testLine = testLine.Trim(); }

            switch (StringSearchOption)
            {
                case StringSearchOption.StartsWith:
                    if (testLine.StartsWith(propertyName.ToUpper())) { results.Add(line); }
                    break;
                case StringSearchOption.Contains:
                    if (testLine.Contains(propertyName.ToUpper())) { results.Add(line); }
                    break;
                case StringSearchOption.EndsWith:
                    if (testLine.EndsWith(propertyName.ToUpper())) { results.Add(line); }
                    break;
            }
        }
    }
    catch
    {
        // Trap any exception that occurs in reading the file and return null.
        results = null;
    }
    finally
    {
        if (sr != null) {sr.Close();}
    }

    return results;
}

After the GetPropertyValue method, add the following code to override the TryGetMembermethod of the DynamicObject class. The TryGetMember method is called when a member of a dynamic class is requested and no arguments are specified. The binder argument contains information about the referenced member, and the result argument references the result returned for the specified member. The TryGetMember method returns a Boolean value that returns true if the requested member exists; otherwise it returns false.
```
		// Implement the TryGetMember method of the DynamicObject class for dynamic member calls.
public override bool TryGetMember(GetMemberBinder binder,
                                  out object result) 
{
    result = GetPropertyValue(binder.Name);
    return result == null ? false : true;
} 
```

After the TryGetMember method, add the following code to override the TryInvokeMembermethod of the DynamicObject class. The TryInvokeMember method is called when a member of a dynamic class is requested with arguments. The binder argument contains information about the referenced member, and the result argument references the result returned for the specified member.

The custom version of the TryInvokeMember method expects the first argument to be a value from the StringSearchOption enum that you defined in a previous step. The TryInvokeMembermethod expects the second argument to be a Boolean value. If one or both arguments are valid values, they are passed to the GetPropertyValue method to retrieve the results.

		// Implement the TryInvokeMember method of the DynamicObject class for 
// dynamic member calls that have arguments.
public override bool TryInvokeMember(InvokeMemberBinder binder,
                                     object[] args,
                                     out object result)
{
    StringSearchOption StringSearchOption = StringSearchOption.StartsWith;
    bool trimSpaces = true;

    try
    {
        if (args.Length > 0) { StringSearchOption = (StringSearchOption)args[0]; }
    }
    catch
    {
        throw new ArgumentException("StringSearchOption argument must be a StringSearchOption enum value.");
    }

    try
    {
        if (args.Length > 1) { trimSpaces = (bool)args[1]; }
    }
    catch
    {
        throw new ArgumentException("trimSpaces argument must be a Boolean value.");
    }

    result = GetPropertyValue(binder.Name, StringSearchOption, trimSpaces);

    return result == null ? false : true;
}

The args argument contains an array of the arguments that are passed to the member. The TryInvokeMember method returns a Boolean value that returns true if the requested member exists; otherwise it returns false.
Save and close the file.

To create a sample text file

Right-click the DynamicSample project and point to Add, and then click New Item. In the Installed Templates pane, select General, and then select the Text File template. Leave the default name of TextFile1.txt in the Name box, and then click Add. A new text file is added to the project.

Copy the following text to the TextFile1.txt file.

		List of customers and suppliers  

Supplier: Lucerne Publishing (https://www.lucernepublishing.com/)  
Customer: Preston, Chris  
Customer: Hines, Patrick  
Customer: Cameron, Maria  
Supplier: Graphic Design Institute (https://www.graphicdesigninstitute.com/)   
Supplier: Fabrikam, Inc. (https://www.fabrikam.com/)   
Customer: Seubert, Roxanne  
Supplier: Proseware, Inc. (http://www.proseware.com/)   
Customer: Adolphi, Stephan  
Customer: Koch, Paul

Save and close the file.

To create a sample application that uses the custom dynamic object

In Solution Explorer, double-click the Module1.vb file if you are using Visual Basic or the Program.cs file if you are using Visual C#.

Add the following code to the Main procedure to create an instance of the ReadOnlyFile class for the TextFile1.txt file. The code uses late binding to call dynamic members and retrieve lines of text that contain the string "Customer".

		dynamic rFile = new ReadOnlyFile(@"..\..\TextFile1.txt");
foreach (string line in rFile.Customer)
{
    Console.WriteLine(line);
}
Console.WriteLine("----------------------------");
foreach (string line in rFile.Customer(StringSearchOption.Contains, true))
{
    Console.WriteLine(line);
}

Save the file and press CTRL+F5 to build and run the application.

Calling a Dynamic Language Library

The next project that you create in this walkthrough accesses a library that is written in the dynamic language IronPython.

To create a custom dynamic class

In Visual Studio, on the File menu, point to New and then click Project.
In the New Project dialog box, in the Project Types pane, make sure that Windows is selected. Select Console Application in the Templates pane. In the Name box, type DynamicIronPythonSample, and then click OK. The new project is created.
If you are using Visual Basic, right-click the DynamicIronPythonSample project and then click Properties. Click the References tab. Click the Add button. If you are using Visual C#, in Solution Explorer, right-click the References folder and then click Add Reference.
On the Browse tab, browse to the folder where the IronPython libraries are installed. For example, C:\Program Files\IronPython 2.6 for .NET 4.0. Select the IronPython.dll, IronPython.Modules.dll, Microsoft.Scripting.dll, and Microsoft.Dynamic.dll libraries. Click OK.
If you are using Visual Basic, edit the Module1.vb file. If you are using Visual C#, edit the Program.cs file.
At the top of the file, add the following code to import the Microsoft.Scripting.Hosting and IronPython.Hosting namespaces from the IronPython libraries.
```
		using Microsoft.Scripting.Hosting;
using IronPython.Hosting;
```

In the Main method, add the following code to create a new Microsoft.Scripting.Hosting.ScriptRuntime object to host the IronPython libraries. The ScriptRuntime object loads the IronPython library module random.py.

		// Set the current directory to the IronPython libraries.
System.IO.Directory.SetCurrentDirectory(
   Environment.GetFolderPath(Environment.SpecialFolder.ProgramFiles) + 
   @"\IronPython 2.6 for .NET 4.0\Lib");

// Create an instance of the random.py IronPython library.
Console.WriteLine("Loading random.py");
ScriptRuntime py = Python.CreateRuntime();
dynamic random = py.UseFile("random.py");
Console.WriteLine("random.py loaded.");

After the code to load the random.py module, add the following code to create an array of integers. The array is passed to the shuffle method of the random.py module, which randomly sorts the values in the array.

		// Initialize an enumerable set of integers.
int[] items = Enumerable.Range(1, 7).ToArray();

// Randomly shuffle the array of integers by using IronPython.
for (int i = 0; i < 5; i++)
{
    random.shuffle(items);
    foreach (int item in items)
    {
        Console.WriteLine(item);
    }
    Console.WriteLine("-------------------");
}

Save the file and press CTRL+F5 to build and run the application.

Walkthrough: Creating and Using Dynamic Objects

Prerequisites

Creating a Custom Dynamic Object

To create a custom dynamic class

To create a sample text file

To create a sample application that uses the custom dynamic object

Calling a Dynamic Language Library

To create a custom dynamic class

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