CODEWITHSUNDEEP
c#
Albert
Albert

Reputation: 41

How to cast string into Dropdownlist?

I have 7 dropdown lists for the the hours of the day for each each day.

Instead of typing datasource, textfield, and valuefield for each drop down, I decided to create a small code that will iterate through an enum and append at the end the day of the week.

However I get an error : Unable to cast object of type 'System.String' to type 'System.Web.UI.WebControls.DropDownList'.

The line ((DropDownList)strTimeFrom).DataSource = TimesAvailable(); Doesn't show any error at run time.

enum Days
{
    Sunday = 1,
    Monday = 2,
    Tuesday = 3,
    Wednesday = 4,
    Thursday = 5,
    Friday = 6
}
protected void Page_Load(object sender, EventArgs e)
{        
    foreach (Days day in Enum.GetValues(typeof(Days)))
    {
        object strTimeFrom = "ddlTimeFrom" + day;
        object strTimeTo = "ddlTimeTo" + day;

        ((DropDownList)strTimeFrom).DataSource = TimesAvailable();
        ((DropDownList)strTimeFrom).DataTextField = "Value";
        ((DropDownList)strTimeFrom).DataValueField = "Key";
        ((DropDownList)strTimeFrom).DataBind();
    }
}

protected List<KeyValuePair<int, string>> TimesAvailable()
{
    List<KeyValuePair<int, string>> lstTime = new List<KeyValuePair<int, string>>();
    lstTime.Add(new KeyValuePair<int, string>(-1, "--Select Time---"));

    for (int intCnt = 1; intCnt <= 12; intCnt++)
    {
        lstTime.Add(new KeyValuePair<int, string>(intCnt, intCnt.ToString()));
    }

    return lstTime;
}

Upvotes: 2

Views: 1632

Answers (3)

sh1rts
sh1rts

Reputation: 1884

You're getting that at runtime because you're trying to cast a string to a DropDownList. You might think that declaring strTimeFrom as object, and appending the day name to get your actual DropDownList, is going to work - but it won't. At the end of the day, strTimeFrom/To are strings and you cannot cast those as DropDownList.

In your case, you should declare these variables as string, and try

var list = Page.FindControl(strTimeFrom) as DropDownList;
list.DataSource = TimesAvailable();
list.DataTextField = "Value";
list.DataValueField = "Key";
list.DataBind();

As a rule you should not declare variables as object, particularly if you really want intrinsic types due to the boxing overhead. C# is a strongly-typed language; you need to be more specific.

Declaring variables as object just to get your code to compile suggests to me you need to do some more reading on C# fundamentals.

Upvotes: 3

System Down
System Down

Reputation: 6270

strTimeFrom is a string (the name of the control I assume) so you can't just cast it into a dropdownlist.

You'll need to get an instance of the control, and since you know it's name you can use FindControl.

var strTimeFrom = (DropDownList)Page.FindControl("ddlTimeFrom" + day);

Upvotes: 8

Softwarehuset
Softwarehuset

Reputation: 815

You cant cast a string to a dropdownlist. Pretty simple

Upvotes: 3

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