If I have a C# method similar to
public void MyMethod(object myObject, string s1, string s2, string s3, ...)
{ ... }
and I needed to do a value check on the strings (if !string.IsNullOrEmpty(var)), instead of going through each variable, because there are quite a few of if checks, is there a more generic way I could accomplish this?
I was hoping to use ParameterInfo until I found you could not retrieve the Value of the parameter. My parameters will either be "" or have a value of a number (as a string) or true/false (as a string) -- these are going into a web.config, hence the string. The name of the method variable is the parameter name going into the web.config. I'm preventing writing of parameters that have null/empty values into the web.config, hence the IsNullOrEmpty check.
Right now what I have for each method parameter is below, just to give you an idea of format.
string name, value;
if (!string.IsNullOrEmpty(s1))
{
name = "s1";
value = s1;
/* do stuff */
}
if (!string.IsNullOrEmpty(s2))
{
name = "s2";
value = s2;
/* do stuff */
}
/* too many of these */
I think that would be better to pass instead of string1, string2, etc.. something like an array of strings (string[]).
This way you will be able to pass 2 arguments, or 10000, and you will not need to check, because in your array you will not include nulls if you want ;)
EDIT: Dictionary wins
public void MyMethod(object myObject, Dictionary<int, string> yourParams)
{ ... }
Related
edit; Based on responses, I may have been unclear in my final goal. I've updated the last section.
Situation
I have a number of variables which I need to perform the same operation on. In this case, they are strings, and can at the point we reach this code have the value null, "", "Blank", or they could already have an assigned other value that I want to keep.
if (String.IsNullOrEmpty(MyVar1) || "Blank".Equals(MyVar1))
MyVar1 = null;
if(String.IsNullOrEmpty(MyVar2) || "Blank".Equals(MyVar2))
MyVar2 = null;
...
if(String.IsNullOrEmpty(MyVar10) || "Blank".Equals(MyVar10))
MyVar10 = null;
Being a programmer that wants to keep my code clean and this block drives me mad, I'm looking for a way to create a list of these variables, and perform this same if statement + null assignment on each.
For an example, here's what I'd like to do:
MyVar1 = "Blank";
DreamDataStructure varList = new DreamDataStructure() { MyVar1, MyVar2, ..., MyVar10 };
foreach(ref string MyVar in varList)
{
if(String.IsNullOrEmpty(MyVar) || "Blank".Equals(MyVar))
MyVar = null;
}
Console.WriteLine(MyVar1); //Should now be null
What Doesn't Work
1) Because my variables are strings, I can't do something like this.
var myListOfVariables = new[] { &MyVar1, &MyVar2, ..., &MyVar10 };
If I could, I'd be able to foreach over them as expected. Because string is a managed type though, it cannot be passed by reference like this.
2) Similarly, if I just made a List<string> of the variables, they would be passed by value and wouldn't help my case.
3) These variables can't be wrapped in an outer object type, as they need to be used as strings in a large number of places in a legacy application. Assume that it would be too large an effort to change how they're used in every location.
Question
Is there a way to iterate over string (or other managed type) variables in a pass-by-reference way that will allow me to put the entire operation inside of a loop and reduce the duplication of code that's happening here?
The goal here is that I can use the original variables later on in my code with the updated values. MyVar1, etc, are referenced later on already by legacy code which expects them to be null or have an actual value.
If I understand your question correctly, I don't think what you want to do is possible. Please see this question: Interesting "params of ref" feature, any workarounds?
The only thing I can suggest (which I know doesn't answer your question) is creating a method to avoid duplication of your conditional logic:
void Convert(ref string text)
{
if (string.IsNullOrEmpty(text) || "Blank".Equals(text))
{
text = null;
}
}
You could create a function instead of passing references, which would also be more readable.
string Validate(string inputString)
{
return string.IsNullOrEmpty(inputString) || "Blank".Equals(inputString) ? null : inputString;
}
<...>
MyVar1 = Validate(MyVar1);
Update:
Now I get what you're trying to do. You have a bunch of variables, and you want to perform some sort of bulk operation on them without changing anything else. Putting them in a class isn't an option.
In that case you're really stuck operating on them one at a time. There are ways to shorten it, but you're pretty much stuck with the repetition.
I'd
create a string SanitizeString(string input) function
type x = SanitizeString(x); once for each variable
copy and paste the variable names to replace x.
It's lame, but that's about all there is.
Perhaps this would be a better approach. It ensures that the values are always sanitized. Otherwise you can't easily tell whether the values have been sanitized or not:
public class MyValues
{
private string _value1;
private string _value2;
private string _value3;
public string Value1
{
get { return _value1; }
set { _value1 = Sanitize(value); }
}
// repeat for other values
private string Sanitize(string input) =>
string.IsNullOrEmpty(input) || string.Equals("Blank", input) ? null : input;
}
That's one option. Another is to sanitize the inputs earlier. But ideally we want to ensure that a given class is always in a valid state. We wouldn't want to have an instance of a class whether the values may or may not be valid. It's better to ensure that they are always valid.
ref doesn't really factor into it. We don't need to use it often, if ever. With a value type or string we can just return a new value from a function.
If we're passing a reference type and we want to make changes to it (like setting its properties, adding items to a list) then we're already passing a reference and we don't need to specify ref.
I'd try to write methods first without using ref and only use it if you need to. You probably never will because you'll succeed at whatever you're trying to do without using ref.
Your comment mentioned that this is a legacy app and it's preferable not to modify the existing class. That leaves one more option - reflection. Not my favorite, but when you say "legacy app" I feel your pain. In that case you could do this:
public static class StringSanitizer
{
private static Dictionary<Type, IEnumerable<PropertyInfo>> _stringProperties = new Dictionary<Type, IEnumerable<PropertyInfo>>();
public static void SanitizeStringProperties<T>(T input) where T : class
{
if (!_stringProperties.ContainsKey(typeof(T)))
{
_stringProperties.Add(typeof(T), GetStringProperties(typeof(T)));
}
foreach (var property in _stringProperties[typeof(T)])
{
property.SetValue(input, Sanitize((string)property.GetValue(input)));
}
}
private static string Sanitize(string input)
{
return string.IsNullOrEmpty(input) || string.Equals("Blank", input) ? null : input;
}
private static IEnumerable<PropertyInfo> GetStringProperties(Type type)
{
return type.GetProperties(BindingFlags.Instance | BindingFlags.Public)
.Where(property => property.PropertyType == typeof(string) && property.CanRead && property.CanWrite);
}
}
This will take an object, find its string properties, and sanitize them. It will store the string properties in a dictionary by type so that once it has discovered the string properties for a given type it won't have to do it again.
StringSanitizer.SanitizeStringProperties(someObject);
you can simply use a string[] and get the changes back to the caller method like this.
public Main()
{
var myVar1 = "Blank";
var myVar2 = "";
string myVar3 = null;
var myVar4 = "";
string[] dreamDataStructure = new string[] { myVar1, myVar2, myVar3, myVar4 };
}
private void ProcessStrings(string[] list)
{
for(int i = 0; i < list.Length; i++)
{
if (String.IsNullOrEmpty(list[i]) || "Blank".Equals(list[i]))
list[i] = null;
}
}
I am hoping that this is a simple question, and it's just my brain that is missing that final link. And if there is another q&a elsewhere, please point me there and close this... but I couldn't find it anywhere.
Here is the gist:
I have a class with a method with optional parameters, something along the lines of
public class Test
{
public void Method(string required, string someoptionalparameter="sometext", string anotheroptionalparameter="someothertext")
{
// do something here with the parameters
}
}
So far, so good.
Now, I am going to instantiate the class and call the method in my code:
...
Test.Method("RequiredString");
and that will work. If I provide optional parameters, it will still work.
But how do I handle a scenario, where I do not know if an optional value is actual provided. So for instance:
...
Test.Method(requiredString,optionalString1,optionalString2);
...
What if I do not know, if the optionalString1 and optionalString2 have a value or not? Do I then need to write an override for every scenario, along the lines of...
if (optionalString1.isEmpty() && optionalString2.isEmpty())
{
Test.Method(requiredString);
}
else if ((!optionalString1.isEmpty() && optionalString2.isEmpty())
{
Test.Method(requiredString, optionalString1);
}
else if...
There has to be another way, and I bet it is simple and I am just having one of those Fridays... Is there something like...
Test.Method(requiredStrinig, if(!optionalString1.isEmpty())...
You should invert the logic - have those optional parameters be null and then do checks in method. So in your case method should be something like:
public void Method(string required, string opt1 = null, string opt2 = null)
{
opt1 = opt1 ?? "some default non-null value if you need it";
opt2 = opt2 ?? "another default value, this one for opt2";
// for those not knowing what it does ?? is basically
// if (opt1 == null) { opt1 = "value"; }
//... rest of method
}
Then calling that method will be easier in outside code and the logic within the method will be able to handle null cases. Outside of method you don't need to worry about those extra parameters, i.e. you can call the method any way you want, like:
Test.Method(requiredString);
Test.Method(requiredString, "something");
Test.Method(requiredString, null, "something else");
Also as #Setsu said in comment you could do this to avoid passing null as second parameter:
Test.Method("required", opt2: "thanks #Setsu");
Use overloads instead, it gives you better semantics and in case you guess the parameters from client code you'd be sure what overload to use, besides, you'll have all the machinery in one place, check out this technique, hope this helps, regards.
class Program {
static void Main(string[] args) {
Work("Hi!");
}
private static void Work(String p1) {
Work(p1, null, null);
}
private static void Work(String p1, String p2) {
Work(p1, p2, null);
}
private static void Work(String p1, String p2, String p3) {
if ( String.IsNullOrWhiteSpace(p2) ) p2 = String.Empty;
if ( String.IsNullOrWhiteSpace(p3) ) p3 = String.Empty;
Console.WriteLine(String.Concat(p1, p2, p3));
}
}
Optional parameters, are, well, optional. They seem to be a way to reduce the number of overloads you have for a method. If you receive all three parameters, you will have to decide if the second or third parameters are empty and need set to their "default" values.
My suggestion is you call your method with three strings and decide in the method if you have to change the values for string two and three. I would probably use a const or readonly instead of the default values.
In the following code, I am trying to check if two strings are anagrams. To that, I am counting the characters in the two strings in a hash table by storing the unique characters as key and its count in the string as values. At the end, when I go to check if each of the character have the same count, I get a false output, see the line marked as "PROBLEM" in the code. But when I convert the values in that line to string, the code works fine. What am I missing?
static bool AreAnagrams(string input1, string input2)
{
Hashtable uniqueChars1 = new Hashtable();
Hashtable uniqueChars2 = new Hashtable();
// Go through first string and create a hash table of characters
AddToHashTable(input1, ref uniqueChars1);
// Go through second string and create a second hash table of characters
AddToHashTable(input2, ref uniqueChars2);
// For each unique character, if the count from both hash tables are the same, they are anagrams
if (uniqueChars1.Keys.Count != uniqueChars2.Keys.Count)
{
return false;
}
else
{
foreach (object key in uniqueChars1.Keys)
{
if (uniqueChars1[key] != uniqueChars2[key]) // ***PROBLEM HERE***
{
return false;
}
}
}
return true;
}
static void AddToHashTable(string input, ref Hashtable uniqueChars)
{
foreach (char c in input)
{
if (!uniqueChars.ContainsKey(c))
{
uniqueChars.Add(c, 1);
}
else
{
int charCount = Convert.ToInt32(uniqueChars[c]);
charCount++;
uniqueChars[c] = charCount;
}
}
}
The Hashtable class is not generic; it just contains Objects, not a specific type.
When you do this:
if (uniqueChars1[key] != uniqueChars2[key])
the compile-time type of uniqueChars[key] is Object, not Int32. So you're using the Object implementation of the inequality operator, which just compares references. Since Int32 is a value type, and the indexer returns an object, the value is boxed; and since you're boxing two values, you get two distinct object instances, so reference equality always returns false.
You have several options:
use a Dictionary<char, int>, which is the generic equivalent of Hashtable
cast the values to int before comparison:
if ((int)uniqueChars1[key] != (int)uniqueChars2[key])
use the Equals method to compare values:
if (!uniqueChars1[key].Equals(uniqueChars2[key]))
Unless you're still using .NET 1.x, I strongly advise you to use generic collections everywhere you can. They're safer, more intuitive, and have better performance for value types.
Side note (unrelated to your problem): you don't need to pass the Hashtable by reference to AddToHashTable; the code would work exactly the same way without the ref modifier, because Hashtable is a reference type, so it's always a reference that is passed anyway. The ref modifier would only be useful if you were assigning something else to the uniqueChars parameter, or if you were passing a value type and mutating its state (which is usually consider a bad thing). I suggest you read Jon Skeet's great article about value types, reference types, and parameter passing.
Your problem comes from the fact that != will compare reference equality on two objects. uniqueCharsX[key] returns a int boxed inside a object, while you have the same int being returned from both hashtables the box they are being returned in is not the same box so you get a incorrect value.
Either use a strongly typed Dictionary<char, int> instead of a hashtable or use !uniqueChars1[key].Equals(uniqueChars2[key]) instead of uniqueChars1[key] != uniqueChars2[key] which will unbox the int and compare on the value instead (I highly recommend you use the Dictionary.)
Is it possible to know the length of a string array - without having an object instance - via reflection?
E.g. in this case: 2.
public string[] Key
{
get { return new string[] { Name, Type }; }
}
EDIT: ok, I will not try to do this, it doesn't make much sense.
Perhaps you mean "without having the exact type of the Array". C# Arrays all derive from Array, so you can cast an Array reference to Array and use the Length property.
If you TRULY wants to reflect the property,
var type = typeof(MyClass);
var prop = type.GetProperty("Key");
var method = prop.GetGetMethod();
var body = method.GetMethodBody();
var ils = body.GetILAsByteArray();
from here you'll have to use one of the various libraries to decode bytes to IL OpCodes (for example https://gist.github.com/104001) . The OpCode you are looking for is newarr. The last push of an int32 before the newarr is the size of the array.
You have two things going on there... telling the length of an array is pretty simple once you have an array; you just call .Length (in the case of a vector).
However, you mention an instance, and you are showing an instance property; which makes me think it is the containing object you lack. In which case... no. You can't make a virtcall on a null instance. And trying to use static-call on an instance member of a class is very evil; IIRC the runtime will kick you for this.
You could, however, make it a static property just by adding the static modifier. Then you just pass in null as the instance to reflection.
I guess you mean you want to know the size of the array the property will return if it were called?
I don't think you can do it sensibly.
If the property had a conditional then it could return different sized arrays, so
you'd have to evaluate the property to know the size. Which could have side effects or be dependent on other values in the object (or statics).
Consider this one:-
static public int n;
public string[] Key
{
get {
if (n > 1)
return new string[] { "Name", "Type" };
else
return new string[] { "Name", "Type", "Cheese" };
}
}
Basically, you'd have to run the code.
When reference variable can be passed by reference :
class Example
{
public string str="Demo";
public int[] intValues={1,3,4,5};
public static void StrPassing(string someStr)
{
string otherStr="Changed!";
someStr=otherStr;
}
public static void NumPassing(int[] a)
{
a[2] = 115;
}
}
static void Main(string[] args)
{
Example ex = new Example();
Example.StrPassing(ex.str);
Example.NumPassing(ex.intValues);
foreach (int i in ex.intValues)
{
Console.WriteLine(i);
}
Console.WriteLine(ex.str);
Console.ReadLine();
}
the value of intValues[2] is changed as 115 as the reference is being passed.But the value of the string "str" (demo) is not changed to "Changed!".What is the reason for it?.can i take it as Arrays are passed by reference and other reference types are passed by
value?
Whatever you pass to a method as arguments is passed by value which, for reference types, means that a reference is passed by value. So you can't change the object to another one but you can surely change its contents (because that doesn't change the actual reference, just some memory elsewhere).
As your example with the array demonstrates you take the array reference (but don't change it) and change a value in the array. This is just like taking some object and changing a property value. You can do this just fine from within a method too.
If you want to change a string, which is an immutable object in .NET, then you need to resort to ref parameters:
public static void StrPassing(ref string someStr)
{
string otherStr="Changed!";
someStr=otherStr;
}
And call it like this:
string foo = "foo";
StrPassing(ref foo);
Console.WriteLine(foo); // should print "Changed!"
The ref keyword ensures that your method gets the actual reference to the string and can change it, instead of just a copy of the reference. So then you can replace the object by an entirely new one.
To come back to your array: You'd have a hard time too, to change the passed array to an entirely different array:
public static void NumPassing(int[] a)
{
a = new int[15];
}
wouldn't work too because then you'd try exactly the same as changing a string to an entirely different string.
You need to distinguish between changing which object a variable refers to and changing *the content of the object".
In this code:
public static void StrPassing(string someStr)
{
string otherStr="Changed!";
someStr=otherStr;
}
... you are changing the value of someStr. You're not making any change to the string that someStr originally refers to. Indeed, you can't because strings are immutable. (If it were a StringBuilder, you could set the length to 0 and then append "Changed!")
Changing the value of someStr has no effect because the argument (ex.str) was passed by value. (The value in question is a reference, but that doesn't mean it's passed by reference.)
Now compare that with this code:
public static void NumPassing(int[] a)
{
a[2] = 115;
}
Here you're not changing the value of a - you're changing the contents of the array that a refers to.
In short, unless you use ref/out, arguments will be passed by value - but for reference types that value is just a reference.
I have an article on parameter passing which goes into all of this in a lot more detail.
What you'd need to do is change the signature for StrPassing to look like this:
public static void StrPassing(ref string someStr)
Strings are special in C#. They are immutable reference types which makes they exhibit similar behavior as value types.
Here's a good discussion.