Use of Dynamic Keyword in .Net 3.5

问题:

I have written this code in visual studio 2013 utilizing .net v4.5. The problem I am having is that I am now having to drop down to .net v3.5 and the dynamic keyword is throwing an error as missing an assembly reference. Is there an equivalent type to 'dynamic' in .net v3.5 or a way for me to achieve the same results as below?

I thought I may have found my answer here, but var is throwing errors when I add the .Attributes or .Text property modifications.

private void CreateControl<T>(string objText,
                              Panel pnl,
                              string HTMLTag = "<td>",
                              string applicantID = "",
                              EventHandler hndl = null)
{
    pnl.Controls.Add(new LiteralControl(HTMLTag));
    dynamic obj = Activator.CreateInstance(typeof(T));
    obj.Text = objText;

    if (applicantID != string.Empty)
    {
        obj.Attributes.Add("ApplicantID", applicantID);
    }
    if (hndl != null)
    {
        obj.Click += new EventHandler(hndl);
    }

    pnl.Controls.Add(obj);
    pnl.Controls.Add(new LiteralControl(HTMLTag.Insert(1, "/")));
}

回答1:

Instead of trying to hack this together in some bound to fail way and since there isn't a 'dynamic' control in .net v3.5, I have instead decided to just completely forgo this method and wrote some overloads instead. This way seems safer at this point; works the same, just a bit more code...

    #region CreateControl() Overloads
            /// <summary>
            /// Creates a LinkButton control.
            /// </summary>
            /// <param name="objText">.Text property of this LinkButton control.</param>
            /// <param name="pnl">Panel this control will be attached to.</param>
            /// <param name="hndl">Event handler attached to this LinkButton control.</param>
            /// <param name="HTMLTag">Opening tag used to contain this control.</param>
            private void CreateControl(string objText,
                                       Panel pnl,
                                       EventHandler hndl,
                                       string HTMLTag)
            {
                pnl.Controls.Add(new LiteralControl(HTMLTag));
                LinkButton obj = new LinkButton();
                obj.Text = objText;
                obj.Click += new EventHandler(hndl);

                pnl.Controls.Add(obj);
                pnl.Controls.Add(new LiteralControl(HTMLTag.Insert(1, "/")));
            }
            /// <summary>
            /// Creates a Label control.
            /// </summary>
            /// <param name="objText">.Text property of this Label control.</param>
            /// <param name="pnl">Panel this control will be attached to.</param>
            /// <param name="HTMLTag">Opening tag used to contain this control.</param>
            private void CreateControl(string objText,
                                       Panel pnl,
                                       string HTMLTag)
            {
                pnl.Controls.Add(new LiteralControl(HTMLTag));
                Label obj = new Label();
                obj.Text = objText;

                pnl.Controls.Add(obj);
                pnl.Controls.Add(new LiteralControl(HTMLTag.Insert(1, "/")));
            }
            /// <summary>
            /// Creates the specified literal control.
            /// </summary>
            /// <param name="ControlText">HTML text containing instructions for creating the desired literal control.</param>
            /// <param name="pnl">Panel this literal control will be attached to.</param>
            private void CreateControl(string ControlText, 
                                       Panel pnl)
            {
                pnl.Controls.Add(new LiteralControl(ControlText));
            }
        #endregion

回答2:

Is there an equivalent type to 'dynamic' in .net v3.5

No. dynamic requires .NET 4.0.

or a way for me to achieve the same results as below?

You could use reflection instead of dynamic to create the control and add your event handlers.

However, since this appears to be one of a few custom controls you're creating (given the attributes, etc), you may be able to constrain to an interface or base class, which would allow you to create the items and use those shared properties directly.

回答3:

Based on your code, it looks like you're writing a generic method to pass in some unknown controls and attach them to a panel.

It also looks like you're dealing with different types of controls; i.e., not all WebControls have Text, and Attributes, AND Click properties;

This is a bit hacky but works in 3.5 - you can just use casting of the various underlying types or interfaces to access the needed properties, something like this:

private void CreateControl<T>(string objText, Panel pnl, string HTMLTag, 
    string applicantID, EventHandler hndl)
        where T : Control, new()
{
    pnl.Controls.Add(new LiteralControl(HTMLTag));
    T obj = new T();

    if (obj is ITextControl) (obj as ITextControl).Text = objText;

    if (applicantID != string.Empty && obj is WebControl)
        (obj as WebControl).Attributes.Add("ApplicantID", applicantID);


    if (obj is IButtonControl)
    {
        (obj as IButtonControl).Text = objText;
        if (hndl != null)
        {
            (obj as IButtonControl).Click += new EventHandler(hndl);
        }
    }

    pnl.Controls.Add(obj as Control);
    pnl.Controls.Add(new LiteralControl(HTMLTag.Insert(1, "/")));
}

Test code:

protected void Page_Load(object sender, EventArgs e)
{
    var panel = new Panel();

    CreateControl<Button>("test", panel, "<td>", "123", (s, args) => Console.WriteLine("hello!"));
    CreateControl<Label>("test", panel, "<td>", "123", (s, args) => Console.WriteLine("hello!"));
    CreateControl<Panel>("test", panel, "<td>", "123", (s, args) => Console.WriteLine("hello!"));
    CreateControl<Literal>("test", panel, "<td>", "123", (s, args) => Console.WriteLine("hello!"));

    //This won't compile because object doesn't fit <control> constraint
    //CreateControl<object>("test", panel, "<td>", "123", (s, args) => Console.WriteLine("hello!"));
}

To be honest I'm not 100% sure I like this approach. I might use some more specific methods and possibly method overloading to get more specific with different types of control creation, but this may help point you in the right direction.

Note that optional parameters are also not yet "invented" in C# 3.0 which shipped with .net 3.5, so you have to actually pass in all of the values.

回答4:

dynamic keyword is available on .net 4.x and is a simple way to store any kind of value, it just resolve his type in runtime. It has been useful to me working with JSON strings.

string jsonValue = "{name:'Pedro',lastName:'Mora'}";
dynamic Variable = new JavascriptSerializer().Deserialize<dynamic>(jsonValue);
return Variable.name;
//It will return "Pedro"

Thing is that you have to be sure that the value won't be null and the properties or attributes or methods or something refered to the object exists and it takes it's values on runtime.