How to define a global variable in ASP.net web app

Another option of defining a global variable is by creating a static class with a static property:

public static class GlobalVariables
{
    public static string MyGlobalVariable { get; set; }
}

You can make this more complex if you are going to use this as a data store, but the same idea goes. Say, you have a dictionary to store your global data, you could do something like this:

public static class GlobalData
{
    private static readonly object _syncRoot = new object();
    private static Dictionary<string, int> _data;

    public static int GetItemsByTag(string tag)
    {
        lock (_syncRoot)
        {
            if (_data == null)
                _data = LoadItemsByTag();

            return _data[tag];
        }
    }

    private static Dictionary<string, int> LoadItemsByTag()
    {
        var result = new Dictionary<string, int>();

        // Load the data from e.g. an XML file into the result object.

        return result;
    }
}

To Share the data with all application users, you can use ASP.NET Application object. Given is the sample code to access Application object in ASP.NET:

Hashtable htblGlobalValues = null;

if (Application["GlobalValueKey"] != null)
{
    htblGlobalValues = Application["GlobalValueKey"] as Hashtable;
}
else
{
    htblGlobalValues = new Hashtable();
}

htblGlobalValues.Add("Key1", "Value1");
htblGlobalValues.Add("Key2", "Value2");

this.Application["GlobalValueKey"] = htblGlobalValues;

Application["GlobalValueKey"] can be used anywhere in the whole application by any user. It will be common to all application users.

You can stuff data into the Application object if you want. It isn't persistent across application instances, but that may sufficient.

(I'm not for a minute going to suggest this is a best practice, but without a clearer picture of the requirements, that's all I can suggest.)

http://msdn.microsoft.com/en-us/library/system.web.ui.page.application.aspx
http://msdn.microsoft.com/en-us/library/system.web.httpapplication.aspx