Is there some kind of equivalent of Python's **kwargs in C#? I would like to be able to pass variable number of named arguments into functon, then get them as something Dictionary-like inside function and cycle over them.
There is nothing in C# available to let you pass in arbitrary named parameters like this.
You can get close by adding a Dictionary<string, object> parameter, which lets you do something similar but requiring a constructor, the "parameter names" to be strings and some extra braces:
static void Method(int normalParam, Dictionary<string, object> kwargs = null)
{
...
}
Method(5, new Dictionary<String, object>{{ "One", 1 }, { "Two", 2 }});
You can get closer by using the ObjectToDictionaryRegistry here, which lets you pass in an anonymous object which doesn't require you to name a dictionary type, pass the parameter names as strings or add quite so many braces:
static void Method(int normalParam, object kwargs = null)
{
Dictionary<string, object> args = ObjectToDictionaryRegistry(kwargs);
...
}
Method(5, new { One = 1, Two = 2 });
However, this involves dynamic code generation so will cost you in terms of performance.
In terms of syntax, I doubt you'll ever be able to get rid of the `new { ... }' wrapper this requires.
If you specifically want a series of KeyValuePairs instead of an array of values, you could do something like this:
public class Foo
{
public Foo(params KeyValuePair<object, object>[] myArgs)
{
var argsDict = myArgs.ToDictionary(k=>k.Key, v=>v.Value);
// do something with argsDict
}
}
myArgs would be an array of KeyValuePair<object, object> that you can iterate or convert to a dictionary as shown above. Just a note though, the conversion to dictionary might fail if you pass multiple KeyValuePair<>s with the same key. You might have to do some checking ahead of time before converting to a dictionary.
You would call it like this:
KeyValuePair<object, object> myKvp1 = new KeyValuePair<object, object>(someKey1, someValue1);
KeyValuePair<object, object> myKvp2 = new KeyValuePair<object, object>(someKey2, someValue2);
KeyValuePair<object, object> myKvp3 = new KeyValuePair<object, object>(someKey3, someValue3);
Foo foo = new Foo(myKvp1, myKvp2, myKvp3);
Yes. The ability to use optional and named parameters was added in, I believe, C# 4.0.
http://msdn.microsoft.com/en-us/library/dd264739(v=vs.100).aspx
void paramsExample(object arg1, object arg2, params object[] argsRest)
{
foreach (object arg in argsRest)
{ /* .... */ }
}
call it like this,
paramsExample(1, 0.0f, "a string", 0.0m, new UserDefinedType());
You can use a param list as your final argument to the function, like this:
void paramsExample(object arg1, object arg2, params object[] argsRest) {
foreach (object arg in argsRest)
{ /* .... */ }
}
the method can be invoked with any number of arguments of any type.
paramsExample(1, 0.0f, "a string", 0.0m, new UserDefinedType());
This is a strongly typed argument so if I wanted to use a list of strings I could do this:
public string Concat(string separator, params string[] strings) {
string result = "";
for (int i = 0; i < strings.Length; i++) {
if (i > 0)
result += separator;
result += strings[i];
}
return result;
}
To invoke:
MessageBox.Show(Concat("+", "Anders", "Eric", "Scott", "Duncan") + " = great team");
Check out more info here:
http://blogs.msdn.com/b/csharpfaq/archive/2004/05/13/how-do-i-write-a-method-that-accepts-a-variable-number-of-parameters.aspx
Related
I have a C# method declared like so:
public void Process<K, V>(params KeyValuePair<K, V>[] items)
{
...
}
Usage of this method looks kind of ugly; for example:
Process(
new KeyValuePair("key", "value"),
new KeyValuePair(123, Guid.NewGuid())
);
In Kotlin, you can create pairs using the to infix function; for example:
val pair1 = "key" to "value"
val pair2 = 123 to UUID.randomUUID()
So the equivalent method usage looks a little tidier; for example:
process("key" to "value", 123 to UUID.randomUUID())
C# doesn't have infix functions, but something nearly equivalent could be achieved with extension methods; for example:
public static KeyValuePair<K, V> To<K, V>(this K key, V value) where K : notnull
{
return new KeyValuePair(key, value);
}
var pair1 = "key".To("value");
var pair2 = 123.To(Guid.NewGuid());
Process("key".To("value"), 123.To(Guid.NewGuid()));
That doesn't seem like the most elegant solution, so the other thing I was considering was that C# has dictionary initializer syntax; for example:
new Dictionary<object, object>()
{
["key"] = "value",
[123] = Guid.NewGuid()
}
or
new Dictionary<object, object>()
{
{ "key", "value" },
{ 123, Guid.NewGuid() }
}
So I was wondering whether dictionary initializer syntax could be applied to a method parameter; for example:
Process({ ["key"] = "value", [123] = Guid.NewGuid() });
or
Process({{ "key", "value" }, { 123, Guid.NewGuid() }});
Questions
Is dictionary initializer syntax as a method parameter possible, or is it syntactic sugar provided by the compiler when using a dictionary?
Are there any other elegant ways to create params of KeyValuePair<K, V>?
This is currently (as of 2023) the best you can get with C# out of the box.
As you wrote yourself, C# 3 introduced shorthand init using curly brackets, C# 6 introduced named parameters init, and C# 9 allows you to use new() when the instantiated type can be inferred.
Meaning that you can do:
// C# 3+
CallMethod(new Dictionary<string, string> { { "ka", "a" }, { "kb", "b" } });
// C# 6+
CallMethod(new Dictionary<string, string> { ["ka"] = "a", ["kb"] = "b" });
// C# 9+
CallMethod(new() { { "ka", "a" }, { "kb", "b" } });
If you really want to use params, one way to reduce some keystrokes would be to use ValueTuples, something like
public static Dictionary<K, V> CallMethod<K, V>(params (K key, V value)[] values)
{
var dict = new Dictionary<K, V>();
foreach (var kvp in values)
{
dict[kvp.key] = kvp.value;
}
return dict;
}
and then you can do
// C# 7+, return type is inferred to be Dictionary<string, string>
CallMethod(("ka", "a"), ("kb", "b"));
I'm trying to convert command line arguments to different formats. For example, if I'm trying to get a specific user, it would be GetUser=UserName, but I also have methods that don't need the equals sign, such as GetAllUsers. Right now I'm currently splitting on the = but I don't need to do that for all commands. I tried setting it to a conditional, where if = is detected, it will split, otherwise it will just take the argument as a string, but I'm getting Cannot implicitly convert type 'string' to 'System.Collections.Generic.Dictionary<string, string>
Code:
public static Dictionary<string, string> ParseArgs(string[] args)
{
Dictionary<string, string> results = new Dictionary<string, string>();
foreach (string arg in args)
{
string[] parts = arg.Split('=');
if (parts.Length > 1)
{
results[parts[0]] = parts[1];
continue;
}
else
{
results = Convert.ToString(arg);
}
}
return results;
}
You're trying to assign a string to a dictionary object. If you want a collection (like Dictionary) then you should use the Add method like so:
foreach (string arg in args)
{
string[] parts = arg.Split('=');
if (parts.Length > 1)
{
//results[parts[0]] = parts[1];
results.Add(parts[0], parts[1]);
continue;
}
else
{
results.Add("GetUser", arg);
}
}
I've forced it to be "GetUser" as the value here, but you may actually want something different. Your dictionary is using Key-Value pairs and you would say results["GetUser"] to return a string for "GetUser". You could also use TryGetValue to validate that GetUser was actually provided.
Additionally, if you just want to return the string that's after GetUser (e.g. Username), then you could change results to a string and just assign that directly and skip the whole use of a Dictionary, which seems overkill for your sample (perhaps your real project is much more complex though).
In the else part, you are trying to assign string to dictionary via the statement results = Convert.ToString(arg);
So, please change else part with something like as shown below:
public static Dictionary<string, string> ParseArgs(string[] args)
{
Dictionary<string, string> results = new Dictionary<string, string>();
foreach (string arg in args)
{
string[] parts = arg.Split('=');
if (parts.Length > 1)
{
results[parts[0]] = parts[1];
// continue; no need to use continue, as there are no more statements after this if-else, loop will anyway continue. but please uncomment if you have any in your actual code.
}
else
{
results[arg] = arg; // assuming arg string in args won't repeat, meaning its unique. Otherwise please use ContaineKey if condition before adding or over-writing.
}
}
return results;
}
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OLD Question
So this is what I am trying to achieve...
I have an existing abstract class ..lets name it Class1.cs . It
contains definitions for a number of methods. So now I have included
some new functionality which needs to be implemented in each and every
method of the Class1 class. So for ex-
public void Method_1(string arg1, string arg2)
{
/*
//some code implementation specific to Method_1
*/
Dictionary<string, object> dict= new Dictionary<string, object>();
//there can be more or less arguments in other methods
dict.Add("Arg1", arg1);
dict.Add("Arg2", arg2);
Method_2(dict);
}
I have to do the exact same thing in all the methods but the arguments
can vary. So the dictionary object can have "n" parameters . Is there
a way that I can avoid the manual labor of adding the same code
repeatedly (maybe use a design pattern if possible)
i think i am not clear... bundling up the dictionary generation
mechanism is not my concern , I would still need to add the same code
in all the methods(around 50) ..I am trying to avoid manually calling
the same code again and again 50 times...
Edited and reframed the question
I have finally decided that I would build the dictionary in a private method and call that in all the other methods. Kindly ignore everything else before this paragraph.
My methods will look like this
public void Method1(string a, string b , string c)
{
Dictionary<string,object> dict = BuildDictionary(new {a, b ,c});
/*the dict object should have the following structure
key=a, value= value of a
key =b , value = value of b
key =b , value = value of b*/
}
public void Method2(string x, string y)
{
Dictionary<string,object> dict = BuildDictionary(new {x,y});
/*the dict object should have the following structure
key= x, value= value of x
key =y , value = value of y */
}
private Dictionary<string,object> BuildDictionary(params object[] values)
{
//values.ToString() gives = { a= "Value of a", b== "Vaue of b", c= "Value of c"}
//Copy pasting Simon's Code here(use of anonymous objects)
var dictionary = values.GetType()
.GetProperties()
.ToDictionary(pi => pi.Name, pi => pi.GetValue(values));
//this dictionary object gives me a count of 7 with keys as the properties of the object datatype(which is not relevant to my case).
}
So what changes do i need to make to the BuildDictionary method in order to get the desired dictionary structure?
Without much information on the nature of Method_2 and what the Keys in the dictionary represents, I'll suggest two options.
The keys always represent Arg + N
In this case, the signature of the Method_2 could be a params
public void Method_2(params object[] values)
{
var argNo = 0;
var dictionary = values.ToDictionary(x => "Arg" + ++argNo);
}
public void Method_1(string arg1, string arg2)
{
// ...
Method_2(arg1, arg2);
}
The keys represent the caller's method parameters name
A generic way to do this would be with anonymous object.
public void Method_2(object values)
{
var dictionary = values.GetType()
.GetProperties()
.ToDictionary(pi => pi.Name, pi => pi.GetValue(values));
}
public void Method_1(string arg1, string arg2)
{
Method_2(new { arg1, arg2 });
}
Edit: If Method_2 cannot be changed either, build the dictionary with a separate method with the same logic as one of the two options.
Answer to edit: The reason your implementation is not working is because you are get all the properties on an array of object and not on a (anonymous) object. You didn't copy my code completely. params object[] values should be object values.
using System.Linq;
...
// add as many elements as you want
var args = new object[] {arg1, arg2, arg3, arg4};
int i = 0;
var dict = args.ToDictionary(x => "Arg" + ++i, x => x);
Method_2(dict);
This will work, but I have no idea why you would want to pass in a dictionary to Method_2 unless you had no other choice.
You should use a loop and have a pattern for how you name your variable. In your method2 you should be able to easily finding these args.
Below, I assume that you have a list of arguments called args, and I add them all to a dictionary. Enjoy
public void Method_1(string arg1, string arg2)
{
/*
//some code implementation specific to Method_1
*/
var dict = GetArguments(args);
Method_2(dict);
}
private Dictionary<string, object> GetArguments(List<string> args)
{
Dictionary<string, object> dict= new Dictionary<string, object>();
var counter = 1;
foreach(var argItem in args)
{
dict.Add("Arg"+counter++, argItem);
}
return dict;
}
Create a private method that will construct the dictionary. You could use a simple loop or or something more concise like Matt suggested.
public void Method_1(string arg1, string arg2)
{
var dict = BuildDictionary(arg1, arg2);
Method_2(dict);
}
private Dictionary<string, object> BuildDictionary(params object[] args)
{
Dictionary<string, object> dict= new Dictionary<string, object>();
for(int i = 0; i < args.Length; i++)
{
dict.Add("Arg" + i, args[i]);
}
return dict;
}
With Matt's version:
private Dictionary<string, object> BuildDictionary(params object[] args)
{
return args.ToDictionary(x => "Arg" + ++i, x => x);
}
I want a function that I can call as an alternative to .ToString(), that will show the contents of collections.
I've tried this:
public static string dump(Object o) {
if (o == null) return "null";
return o.ToString();
}
public static string dump<K, V>(KeyValuePair<K, V> kv) {
return dump(kv.Key) + "=>" + dump(kv.Value);
}
public static string dump<T>(IEnumerable<T> list) {
StringBuilder result = new StringBuilder("{");
foreach(T t in list) {
result.Append(dump(t));
result.Append(", ");
}
result.Append("}");
return result.ToString();
}
but the second overload never gets called. For example:
List<string> list = new List<string>();
list.Add("polo");
Dictionary<int, List<string>> dict;
dict.Add(1, list);
Console.WriteLine(dump(dict));
I'm expecting this output:
{1=>{"polo", }, }
What actually happens is this:
dict is correctly interpreted as an IEnumerable<KeyValuePair<int, List<string>>>, so the 3rd overload is called.
the 3rd overload calls dump on a KeyValuePair>. This should(?) invoke the second overload, but it doesn't -- it calls the first overload instead.
So we get this output:
{[1=>System.Collections.Generic.List`1[System.String]], }
which is built from KeyValuePair's .ToString() method.
Why isn't the second overload called? It seems to me that the runtime should have all the information it needs to identify a KeyValuePair with full generic arguments and call that one.
Generics is a compile time concept, not run time.
In other words the type parametes are resolved at compile time.
In your foreach you call dump(t) and t is of type T.
But there is nothing known about T at this point other than that it is an Object.
That's why the first overload is called.
(updated) As mentioned in other answers, the problem is that the compiler does not know that type V is actually a List<string>, so it just goes to dump(object).
A possible workaround might be to check types at run time. Type.IsGenericType will tell you if the type of a variable has generics or not, and Type.GetGenericArguments will give you the actual type of those generics.
So you can write a single dump method receiving an object and ignoring any generics info. Note that I use the System.Collections.IEnumerable interface rather than System.Collections.Generics.IEnumerable<T>.
public static string dump(Object o)
{
Type type = o.GetType();
// if it's a generic, check if it's a collection or keyvaluepair
if (type.IsGenericType) {
// a collection? iterate items
if (o is System.Collections.IEnumerable) {
StringBuilder result = new StringBuilder("{");
foreach (var i in (o as System.Collections.IEnumerable)) {
result.Append(dump(i));
result.Append(", ");
}
result.Append("}");
return result.ToString();
// a keyvaluepair? show key => value
} else if (type.GetGenericArguments().Length == 2 &&
type.FullName.StartsWith("System.Collections.Generic.KeyValuePair")) {
StringBuilder result = new StringBuilder();
result.Append(dump(type.GetProperty("Key").GetValue(o, null)));
result.Append(" => ");
result.Append(dump(type.GetProperty("Value").GetValue(o, null)));
return result.ToString();
}
}
// arbitrary generic or not generic
return o.ToString();
}
That is: a) a collection is iterated, b) a keyvaluepair shows key => value, c) any other object just calls ToString. With this code
List<string> list = new List<string>();
list.Add("polo");
Dictionary<int, List<string>> dict = new Dictionary<int, List<string>>() ;
dict.Add(1, list);
Console.WriteLine(dump(list));
Console.WriteLine(dump(dict.First()));
Console.WriteLine(dump(dict));
you get the expected output:
{marco, }
1 => {marco, }
{1 => {marco, }, }
To call the second version in your foreach, you need to specify the template parameters K and V, otherwise it will always call the first version:
dump(t); // always calls first version
dump<K,V>(t); // will call the second
How you get the parameter types K and V is another question....
In javascript, I often use something like this object creator
var o = new Object(); // generic object
var props = { color:"red",value:5 }; // hashtable
for(var key in props) o[key] = props[key];
alert(o.color); //red
Can it be written as C# method with this declaration?
static void properties(out Object o, HashTable h) { ...
Is this some design pattern? Am I inventing wheel?
You may want to look at the Expando Object in C# 4. That is about as close as you are going to get to a dynamic object in C# like you can get in JavaScript.
http://msdn.microsoft.com/en-us/magazine/ff796227.aspx
http://www.c-sharpcorner.com/blogs/BlogDetail.aspx?BlogId=2134
var test = new { name = "Testobj", color = Colors.Aqua };
MessageBox.Show(test.name);
It's called a anonymous Type, I think this is what you are searching for.
Since c# is statically typed, you cannot achieve this.. closest possible is anonymous methods
Dictionary<string,int> dic=new Dictionary<string,int>();
dic.Add("red", 5);
dic.Add("black", 10);
dic.Add("white", 1);
object[] obj;
foreach(var o in dic)
{
var sam= new { color = o.Key,value=o.Value };
Console.WriteLine(sam.color);
}