I would like to have a dictionary object contains string keys and values, which are generated from the database. But, I only need to run once when page loads. (C# .NET)
In which way can I do that? I tried the following:
public static class GlobalVar
{
static Dictionary<string, string> GenerateLoginStatus(Dictionary<string, string> List)
{
string Query = "SELECT * FROM LoginStatus";
DataTable Types = MyAdoHelper.ExecuteDataTable(GlobalVar.dbName, Query);
foreach (DataRow Row in Types.Rows)
{
List.Add(Row["Status_Title"].ToString(), Row["Status_Info"].ToString());
}
return List;
}
public const string GlobalString = "ProjectDatabase.mdf";
public static Dictionary<string, string> LoginTypes = GenerateLoginStatus(LoginTypes);
}
Table structure:
Status_Title Status_Info
AlreadyLoggedIn User is already logged in.
A B
C D
And I use it in another page: GlobalVar.LoginStatus["AlreadyLoggedIn"]
The "AlreadyLoggedIn" is created in the database for sure, but when I call any of the keys, it returns an exception (doesn't tell which exception).
EDIT: I changed the code a bit, and it now gives "The type initializer for 'GlobalVar' threw an exception."
I suppose you might be getting null reference exception which is caused because you call your static initialization method with instance you want to initialize (LoginTypes):
public static Dictionary<string, string> LoginTypes =
GenerateLoginStatus(LoginTypes);
When GenerateLoginStatus executes, LoginTypes is null. Yet you attempt to add items to it:
static Dictionary<string, string> GenerateLoginStatus(
Dictionary<string, string> List)
{
// ...
List.Add(Row["Status_Title"].ToString(), Row["Status_Info"].ToString());
This causes null reference exception. You should use type constructor and create that dictionary there:
static GlobalVar()
{
LoginTypes = new Dictionary<string, string>();
// proceed with initialization
}
Related
I'd like to create a Dictionary object, with string Keys, holding values which are of a generic type. I imagine that it would look something like this:
Dictionary<string, List<T>> d = new Dictionary<string, List<T>>();
And enable me to add the following:
d.Add("Numbers", new List<int>());
d.Add("Letters", new List<string>());
I know that I can do it for a list of strings, for example, using this syntax:
Dictionary<string, List<string>> d = new Dictionary<string, List<string>>();
d.Add("Key", new List<string>());
but I'd like to do it for a generic list if possible...
2 questions then:
Is it possible?
What's the syntax?
EDIT: Now I've reread the question...
You can't do this, but a custom collection would handle it to some extent. You'd basically have a generic Add method:
public void Add<T>(string key, List<T> list)
(The collection itself wouldn't be generic - unless you wanted to make the key type generic.)
You couldn't extract values from it in a strongly typed manner though, because the compiler won't know which type you've used for a particular key. If you make the key the type itself, you end with a slightly better situation, but one which still isn't supported by the existing collections. That's the situation my original answer was responding to.
EDIT: Original answer, when I hadn't quite read the question correctly, but which may be informative anyway...
No, you can't make one type argument depend on another, I'm afraid. It's just one of the things one might want to express in a generic type system but which .NET's constraints don't allow for. There are always going to be such problems, and the .NET designers chose to keep generics relatively simple.
However, you can write a collection to enforce it fairly easily. I have an example in a blog post which only keeps a single value, but it would be easy to extend that to use a list.
Would something like this work?
public class GenericDictionary
{
private Dictionary<string, object> _dict = new Dictionary<string, object>();
public void Add<T>(string key, T value) where T : class
{
_dict.Add(key, value);
}
public T GetValue<T>(string key) where T : class
{
return _dict[key] as T;
}
}
Basically it wraps all the casting behind the scenes for you.
How about Dictionary<string, dynamic>? (assuming you're on C# 4)
Dictionary<string, dynamic> Dict = new Dictionary<string, dynamic>();
Source: https://stackoverflow.com/a/5038029/3270733
I prefer this way of putting generic types into a collection:
interface IList
{
void Add (object item);
}
class MyList<T> : List<T>, IList
{
public void Add (object item)
{
base.Add ((T) item); // could put a type check here
}
}
class Program
{
static void Main (string [] args)
{
SortedDictionary<int, IList>
dict = new SortedDictionary<int, IList> ();
dict [0] = new MyList<int> ();
dict [1] = new MyList<float> ();
dict [0].Add (42);
dict [1].Add ("Hello"); // Fails! Type cast exception.
}
}
But you do lose the type checks at compile time.
I came to a type safe implementation using ConditionalWeakTable.
public class FieldByType
{
static class Storage<T>
where T : class
{
static readonly ConditionalWeakTable<FieldByType, T> table = new ConditionalWeakTable<FieldByType, T>();
public static T GetValue(FieldByType fieldByType)
{
table.TryGetValue(fieldByType, out var result);
return result;
}
public static void SetValue(FieldByType fieldByType, T value)
{
table.Remove(fieldByType);
table.Add(fieldByType, value);
}
}
public T GetValue<T>()
where T : class
{
return Storage<T>.GetValue(this);
}
public void SetValue<T>(T value)
where T : class
{
Storage<T>.SetValue(this, value);
}
}
It can be used like this:
/// <summary>
/// This class can be used when cloning multiple related objects to store cloned/original object relationship.
/// </summary>
public class CloningContext
{
readonly FieldByType dictionaries = new FieldByType();
public void RegisterClone<T>(T original, T clone)
{
var dictionary = dictionaries.GetValue<Dictionary<T, T>>();
if (dictionary == null)
{
dictionary = new Dictionary<T, T>();
dictionaries.SetValue(dictionary);
}
dictionary[original] = clone;
}
public bool TryGetClone<T>(T original, out T clone)
{
var dictionary = dictionaries.GetValue<Dictionary<T, T>>();
if (dictionary == null)
{
clone = default(T);
return false;
}
return dictionary.TryGetValue(original, out clone);
}
}
See also this question where the type of the values is stored in as a generic parameter of the keys.
We're using lots of reflection to create an extensible administration tool. We needed a way to register items in the global search in the module definition. Each search would return results in a consistent way, but each one had different dependencies. Here's an example of us registering search for a single module:
public void ConfigureSearch(ISearchConfiguration config)
{
config.AddGlobalSearchCallback<IEmploymentDataContext>((query, ctx) =>
{
return ctx.Positions.Where(p => p.Name.Contains(query)).ToList().Select(p =>
new SearchResult("Positions", p.Name, p.ThumbnailUrl,
new UrlContext("edit", "position", new RouteValueDictionary(new { Id = p.Id }))
));
});
}
In the background during module registration, we iterate over every module and add the Func to a SearchTable with an instance of:
public class GenericFuncCollection : IEnumerable<Tuple<Type, Type, Object>>
{
private List<Tuple<Type, Type, Object>> objects = new List<Tuple<Type, Type, Object>>();
/// <summary>
/// Stores a list of Func of T where T is unknown at compile time.
/// </summary>
/// <typeparam name="T1">Type of T</typeparam>
/// <typeparam name="T2">Type of the Func</typeparam>
/// <param name="func">Instance of the Func</param>
public void Add<T1, T2>(Object func)
{
objects.Add(new Tuple<Type, Type, Object>(typeof(T1), typeof(T2), func));
}
public IEnumerator<Tuple<Type, Type, object>> GetEnumerator()
{
return objects.GetEnumerator();
}
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
return objects.GetEnumerator();
}
}
Then when we finally call it, we do it with reflection:
var dependency = DependencyResolver.Current.GetService(search.Item1);
var methodInfo = search.Item2.GetMethod("Invoke");
return (IEnumerable<SearchResult>)methodInfo.Invoke(search.Item3, new Object[] { query, dependency });
I didn't find what I was looking for here but after reading I think it might be what is being asked for so an attempt to answer.
The problem is that when you use Dictionary it is a closed constructed type and all elements must be of the TValue type. I see this question in a number of places without a good answer.
Fact is that I want indexing but each element to have a different type and based on the value of TKey we already know the type. Not trying to get around the boxing but trying to simply get more elegant access something like DataSetExtensions Field. And don't want to use dynamic because the types are known and it is just not wanted.
A solution can be to create a non generic type that does not expose T at the class level and therefore cause the TValue part of the dictionary to be closed constructed. Then sprinkle in a fluent method to help initialization.
public class GenericObject
{
private object value;
public T GetValue<T>()
{
return (T)value;
}
public void SetValue<T>(T value)
{
this.value = value;
}
public GenericObject WithValue<T>(T value)
{
this.value = value;
return this;
}
}
class Program
{
static void Main(string[] args)
{
Dictionary<string, GenericObject> dict = new Dictionary<string, GenericObject>();
dict["mystring"] = new GenericObject().WithValue<string>("Hello World");
dict["myint"] = new GenericObject().WithValue<int>(1);
int i = dict["myint"].GetValue<int>();
string s = dict["mystring"].GetValue<string>();
}
}
Other posibility it's to use the variable dynamic.
For example:
Dictionary<string, List<dynamic>> d = new Dictionary<string, List<dynamic>>();
d.Add("Key", new List<dynamic>());
the variable dynamic resolve the type on runtime.
No, but you can use object instead of generic type.
Long answer:
The current version of C# will not allow you to make entries of generic type in a dictionary. Your options are either a) create a custom class that is the same as a dictionary except allow it to accept generic types, or b) make your Dictionary take values of type object. I find option b to be the simpler approach.
If you send lists of specific types, then when you go to process the lists you will have to test to see what kind of list it is. A better approach is to create lists of objects; this way you can enter integers, strings, or whatever data type you want and you don't necessarily have to test to see what type of object the List holds. This would (presumably) produce the effect you're looking for.
Here is a short console program that does the trick:
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace dictionary
{
class Program
{
static void Main(string[] args)
{
Dictionary<string, object> dic = new Dictionary<string, object>();
var lstIntList = new List<object>();
var lstStrings = new List<object>();
var lstObjects = new List<object>();
string s = "";
lstIntList.Add(1);
lstIntList.Add(2);
lstIntList.Add(3);
lstStrings.Add("a");
lstStrings.Add("b");
lstStrings.Add("c");
dic.Add("Numbers", lstIntList);
dic.Add("Letters", lstStrings);
foreach (KeyValuePair<string, object> kvp in dic)
{
Console.WriteLine("{0}", kvp.Key);
lstObjects = ((IEnumerable)kvp.Value).Cast<object>().ToList();
foreach (var obj in lstObjects)
{s = obj.ToString(); Console.WriteLine(s);}
Console.WriteLine("");
}
Console.WriteLine("");
Console.WriteLine("press any key to exit");
Console.ReadKey();
}//end main
}
}
One of the way is to create a Dictionary value with type "object" like:
Dictionary<string, object> d = new Dictionary<string, object>();
So, here object datatype is used as a generic datatype, you can put anything in this as a value.
Or it's possible to use generic Type like this
public static void SafeUpdateInDictionary<T, L>(T DictionaryToBeUpdated, string Key, L Value) where T : Dictionary<string, L>
{
if (DictionaryToBeUpdated != null)
{
if(Value != null)
{
if (!DictionaryToBeUpdated.ContainsKey(Key))
DictionaryToBeUpdated.Add(Key, Value);
else
DictionaryToBeUpdated[Key] = Value;
}
}
}
I have a common method which accepts any paramater as object
public static object Populate(object anyStuff)
{
}
I to send a List of a user defined class to this. and then de serialize it again in the method to loop through it. I am not sure how to accomplish.
You can use Generics.
Where you can create the generic method that handles different type of list and operates on them:
public static T Populate<T>(List<T> list){
//Do stuff
}
But if you are using talking about object (do note, not a good way you are trying to achieve):
public static object Populate(object anyStuff){
//Explicit typecast
List<string> value = anyStuff as List<string>();
}
If you want to know the usage of the first example with generic, here you can see the example
Usually you would create multiple overloads of Populate.
If I read your question correctly, you'd like something like this:
var properties = new Dictionary<string, object>
{
{"Name", "Bart"},
{"Date", new DateTime(2014,9,23)},
{"Age", 300}
}
var item = HelperClass.Populate(properies)
Console.WriteLine(item.Name); // should print "bart"
If this is what you want, you should look into the dynamic ExpandoObject.
(see http://blog.jorgef.net/2011/06/converting-any-object-to-dynamic.html )
public static dynamic Populate(IDictionary<string, object> anystuff)
{
IDictionary<string, object> expando = new ExpandoObject()
foreach(var key in anystuff)
{
expando[key] = anystuff[key];
}
return expando;
}
I have 2 data structures: Dictionary<string, string> and Multimap<string, string>.
Multimap is really just a Dictionary under the hood. I took must of the code from this question. Here's the class definition:
public class Multimap<TKey, TValue> : Dictionary<TKey, HashSet<TValue>>
{ ... }
Both data structures have a .Add(TKey key, TValue value) method.
I have a class that is responsible for populating these maps from certain files. I currently have the following two methods:
public Dictionary<string, string> PopulateDictionary(...)
{
Dictionary<string, string> returnDictionary = new Dictionary<string, string>();
...
foreach (...)
{
...
returnDictionary.Add(key, value);
}
return returnDictionary;
}
public Multimap<string, string> PopulateMultimap(...)
{
Multimap<string, string> returnMultimap = new Multimap<string, string>();
...
foreach (...)
{
...
returnMultimap.Add(key, value);
}
return returnMultimap;
}
As you can see, they're exactly the same, both around 25 lines long, and the only difference is their return type. What I am looking to do is condense this into one method.
My first attempt was to have the method
public Dictionary<string, object> PopulateGenericDictionary(...)
{ ... }
Where object was either string or HashSet<string>. But I didn't have much luck casting from Dictionary<string, object> to Multimap<string, string>.
Extracting the logic out of the methods is an option, but it's not great. Because of the foreach loops, there's always going to be some logic inside the two methods. You do end up with methods that are twice as small, but there's still two identical methods, which doesn't truly solve the problem.
This would be my ideal method structure:
public Dictionary<string, string> PopulateDictionary(...)
{
return MethodThatDoesAllTheLogic(...);
}
public Multimap<string, string> PopulateMultimap(...)
{
return MethodThatDoesAllTheLogic(...);
}
public ??? MethodThatDoesAllTheLogic(...)
{ ... }
I've been fiddling around with casting and generics, but I just can't get it to work. Any ideas?
Edit
I have used millimoose's solution. Here's my code now:
public Dictionary<string, string> GenerateDictionary(...)
{
Dictionary<string, string> returnMap = new Dictionary<string, string>();
PopulateDictionary(returnMap.Add, ...);
return returnMap;
}
public Multimap<string, string> GenerateMultimap(...)
{
Multimap<string, string> returnMap = new Multimap<string, string>();
PopulateDictionary(returnMap.Add, ...);
return returnMap;
}
private static void PopulateGenericDictionary(Action<string, string> addFunc, ...)
{
...
foreach (...)
{
addFunc(key, value);
}
}
Much cleaner!
To work around the lack of a common interface, you can invent one ad-hoc using a bunch of delegate type parameters:
void MethodThatDoesAllTheLogic(Action<string, string> addFunc)
{
// ...
addFunc(key, value);
// ...
}
public Dictionary<...> PopulateDictionary()
{
// ...
MethodThatDoesAllTheLogic(result.Add);
}
(Adding more parameters as necessary.)
I would avoid having the helper method create the actual collection at all; have it just populate an existing collection. That can be done much more effectively, since the Add method has the same signature in both cases. We can just use a delegate to accept the Add method:
public static void PopulateMapping<TKey, TValue>(Action<TKey, TValue> addMethod,
IEnumerable<TKey> data) //include other parameters needed to populate the data
{
foreach (var key in data)
{
addMethod(key, default(TValue));
}
}
Then it would be used like this:
public static Dictionary<string, string> PopulateDictionary()
{
Dictionary<string, string> output = new Dictionary<string, string>();
PopulateMapping<string, string>(output.Add, new string[] { "a" });
return output;
}
If you are only looking for an Add method, then both objects should share IDictionary. However, that Add method only uses objects. That is probably the closest that you can get without having to use generics in the method...but again you lose the benefits of generics at that point.
See if this approach will be useful:
The key is to make abstraction on creation of the object (Dictionary or Multimap) and aquiring the values - the two differences in the populating method.
public Dictionary<string, TValue> Populate<TValue>( Dictionary<string, TValue> returnDict, Func<SomeType, TValue> valueProvider)
{
string key = null;
...
foreach (...)
{
...
returnDict.Add(key, valueProvider(value));
}
return returnDict;
}
The example invocation is can be:
public void Test()
{
Populate(new Multimap<string, HashSet<string>>(), (t) => new HashSet<HashSet<string>>());
}
I'm not sure if the valueProvider delegate will be suited to your problem. Try to give more information about it.
If your inner logic is truly identical except for what type TValue is - and I mean word-for-word identical - then you could do something like:
IDictionary<string, TValue> MethodThatDoesAllTheLogic<TValue>(whatever)
{
// word for word-identical logic
}
I made the method take TValue as its only type parameter because that's the only difference (in the example you showed): both methods have string as the first type parameter.
ETA: This assumes that MultiMap implements IDictionary<K,V>. Since you said that it inherited from Dictionary<K,V> I assumed that it did.
in C# with generics you can require them to extend or implement a specific class in our case Dictionary, the following is how you might achieve that.
public T Populate<T>(string val) where T : Dictionary<string, string>, new()
{
T returnDict = new T();
returnDict.Add("key", "val");
return returnDict;
}
I would like to have my comparison functions accessible by using a key. This is what I have tried. I think my problem is how I'm declaring the type for the dictionary value.
internal static int CompareUsersByEmail(MembershipUser a, MembershipUser b)
{
// imagine null checking happening here.
return string.Compare(a.Email, b.Email, true);
}
public static void Sort(List<MembershipUser> list, string expression) {
// I can do this
list.Sort(CompareUsersByEmail);
// but not this
Dictionary<string, Func<MembershipUser, MembershipUser, int>> compareFns;
compareFns = new Dictionary<string, Func<MembershipUser, MembershipUser, int>>();
compareFns["Email"] = CompareUsersByEmail;
list.Sort(compareFns[expression]); // where expression would be "Email"
}
Is this possible?
Change your dictionary type to:
Dictionary<string, Comparison<MembershipUser>> compareFns;
I believe it will then work with no other changes. The method group conversion to Comparison<MembershipUser> will still work, and now you can call List<T>.Sort(Comparison<T>).
I saw this article on msdn with the example http://msdn.microsoft.com/en-us/library/xfhwa508.aspx
So i decided to give it a shot and try this out in my wpf application:
Dictionary<string, string> Dictionarycheck =
new Dictionary<string, string>();
Dictionarycheck.Add("demo1");
Why this won't work? I get the error: Invalid token '(' in class, struct, or interface member declaration
Two problems:
You can't just add a key to a dictionary. You have to add a key/value pair
You can't include statements directly in a class declaration - they have to be in constructors/methods/properties/etc. This is the direct cause of your problem, given your error message.
In other words, you've probably got something like this:
public class Test
{
Dictionary<string, string> Dictionarycheck =
new Dictionary<string, string>();
Dictionarycheck.Add("demo1");
}
when it should be something like this:
public class Test
{
public void DemoMethod()
{
Dictionary<string, string> dictionaryCheck =
new Dictionary<string, string>();
dictionaryCheck.Add("demo1", "value1");
}
}
(I've adjusted the name of the variable for convention, too.)
Dictionary(TKey, TValue)
So its Dictionarycheck.Add("Key", "Value");
You are probably writing the code outside of a method (like I just did to test it). Further, Dictionary.Add has two arguments.