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how to protect static int in C#

I am learning C# so I am still on the basics here. This is my code
class foo
{
protected int id;
protected string data;
static int nextId = 1;
public int Id
{
get { return id; }
}
public foo()
{
this.id = nextId++;
}
public foo(string somedata)
{
this.data = somedata;
this.id = nextId++;
}
}
This code works just fine for now. All objects will have a unique ID with them.
Problem is: I don't want the first ID to be 1, I want it to be the number on the first line of the file given as an argument to the application from the commandline. If no file is specified or the file does not exist, then it can be one. Is there a way to make a one-time method to set nextId so it cannot be tampered with outside of the class after it has been once set?
Duh... I guess this is the trick. Still, I'd like to know if there is some build in way to make variables that can only be set once.
public int nextId
{
set {if(nextId<1) nextId = value;}
}

You can implement static constructor, which assigns proper value to nextId before first usage of foo class:
using System.IO;
using System.Linq;
...
class foo
{
...
static int nextId;
// read-only property which however can be assigned in the constructor
public int Id {get;}
// This static costructor will be called once before 1st usage of foo class
static foo() {
//TODO: put the right file name here
string fileName = Environment.GetCommandLineArgs()[1];
nextId = int.Parse(File
.ReadLines(fileName)
.First());
}
public foo()
{
//Let's increment nextId in thread-safe manner
Id = Interlocked.Increment(ref nextId);
}
...
}

You can do this
class foo
{
private static int nextId = 1;
private static bool isNextIdSet;
public SetId(newId)
{
if (!isNextIdSet) nextId = newId;
isNextIdSet = true;
}
}
So the nextId can be set only once from outside the class.
UPDATE: if you prefer to use this with many variables, you can create a helper for this, such as
class SetOnlyOnce<T>
{
private bool isSet;
public T Value
{
get;
set
{
if (!isSet) Value = value;
isSet = true;
}
}
}
Then use it as
class Foo
{
public SetOnlyOnce<int> prop1 {get;set;}
public SetOnlyOnce<string> prop2 {get;set;}
}
class Bar
{
public Bar()
{
var foo = new Foo();
foo.prop1.Value = 2;
foo.prop1.Value = 3; // this doesn't set.
}
}

As an alternative to Dmitry's answer, you can define the initialization through a static method. I prefer avoiding static constructors unless multiple fields need to be initialized simultaneously.
using System.IO;
using System.Linq;
...
class foo
{
...
// The initialization will be called at some time before first use of nextId.
private static int nextId = GenerateInitialNextId();
// read-only property which however can be assigned in the constructor
public int Id {get;}
private static int GenerateInitialNextId() {
//TODO: put the right file name here
string fileName = Environment.GetCommandLineArgs()[1];
return int.Parse(File
.ReadLines(fileName)
.First());
}
public foo()
{
//Let's increment nextId is thread-safe manner
Id = Interlocked.Increment(ref nextId);
}
...
}

How to acess variable value from one class to another class?

I want to access a string from one class to another. I have used the property method as follows -
Myclass.cs
public class MyClass
{
private string _user;
public string user
{ get { return this._user; } set { this._user = value; } }
}
consumption.aspx.cs
I am assigning the value to user in a function
MyClass m = new MyClass();
m.user = "abc"
Now when I try to use this value in my another function which is called after this value is assigned
RawDal.cs
MyClass m = new MyClass();
string x = m.user;
I get empty value... How to do it?

As already mentioned in the comments you are creating two separate instances of MyClass which results simplified in something like:
int a;
a = 3;
int b;
Console.WriteLine("a: " + b); //<-- here it should be obvious why b is not 3
You can work around this in 3 ways:
1) Use the same instance of MyClass for the second call, but in this case you need to be in the same scope or pass the instance on to the new scope.
2) Make the property/member static:
public class MyClass
{
public static string User { get; set; } //the "static" is the important keyword, I just used the alternative property declaration to keep it shorter
}
Then you can access the same User value everywhere via MyClass.User.
3) Use a singleton:
public class MyClass
{
private static MyClass instance = null;
public static MyClass Instance
{
get
{
if(instance == null)
instance = new MyClass();
return instance;
}
}
public string User { get; set; }
}
Then you can access it via MyClass.Instance.User.
There are possibly some more solutions, but these are the common ones.

You are not using the same instance. Try
public class MyClass
{
private string _user;
public string user
{ get { return this._user; } set { this._user = value; } }
}
public string YourFunction()
{
MyClass m = new MyClass();
m.user = "abc"
return m.user;
}
If all you want to return is a string try something like
string x = YourFunction();

Automatically testing property getters/setters

We use backing fields for a lot of properties on our domain objects, for example:
protected string _firstname;
public virtual string Firstname
{
get { return _firstname; }
set { _firstname = value; }
}
I've occasionally made stupid typos like the example below, and would like to write a single test that verifies all these properties, rather than manually doing a test per object.
public virtual string Firstname
{
get { return _firstname; }
set { _firstname = Firstname; }
}
Would it be easy to write or does a library already exist to test these backing fields get/set correctly? This would only run on properties with setters and (presumably) a backing field that matches the property name using camel-case underscore

Another solution would be to use automatic properties to eliminate this problem:
public virtual string FirstName { get; set; }
UPDATE (see comments, backing field seems needed):
Another possibility is to generate the pocos. Simple t4-template 'Person.tt'
<## template language="C#" #>
<# var pocos = new [] {
Tuple.Create("FirstName", "string"),
Tuple.Create("LastName", "string"),
Tuple.Create("Age", "int")}; #>
public partial class Person {
<# foreach(var t in pocos) {#>
protected <#= t.Item2#> _<#= t.Item1.ToLowerInvariant()#>;
public virtual <#= t.Item2#> <#= t.Item1#>
{
get { return _<#= t.Item1.ToLowerInvariant()#>; }
set { _<#= t.Item1.ToLowerInvariant()#> = value; }
}
<#}#>
}
Now this could of course bring with it as many problems as it solves but it may be worth looking at ... maybe:)

Apart from using auto properties I would think of using reflection to test out my models.. .
Just write a generic method that gets all the properties of your class and then use methods like these:
/ get value of property: public double Number
double value = (double)numberPropertyInfo.GetValue(calcInstance, null);
[C#]
// set value of property: public double Number
numberPropertyInfo.SetValue(calcInstance, 10.0, null);
For your example:
void Main()
{
const int testValue=5;
var test = (Test)Activator.CreateInstance(typeof(Test));
PropertyInfo valuePropertyInfo = typeof(Test).GetProperty("Value");
valuePropertyInfo.SetValue(test, testValue, null);
int value = (int)valuePropertyInfo.GetValue(test, null);
Console.Write(value); //Assert here instead
}
public class Test
{
private int _value;
public int Value {get {return _value;} set{_value=Value;}}
}
the output of the above function is 0 instead of a 5 which is expected. asserting here would have thrown an error.
What do you think about this approach.

Gallio/MbUnit has a contract verifier which does exactly what you are looking for. A typical usage of AccessContract is the following:
public class Foo // Dummy reference type.
{
private readonly int value;
public int Value { get { return value; } }
public Foo (int value)
{
this.value = value;
}
}
public class Bar
{
private Foo foo;
public Bar(string unusedParameter) { }
public Foo Foo // A complex property to be tested!
{
get { return foo; }
set
{
if (value == null)
throw new ArgumentNullException("value");
if (value.Value < 0)
throw new ArgumentOutOfRangeException();
if (value.Value == 666)
throw new ArgumentException("Inferno value strictly forbidden.");
foo = value;
}
}
}
And the test fixture which uses AccessorContract to run various tests on the property.
[TestFixture]
public class BarTest
{
[VerifyContract]
public readonly IContract AccessorTests = new AccessorContract<Bar, Foo>
{
Getter = target => target.Foo,
Setter = (target, value) => target.Foo = value,
ValidValues = { new Foo(123), new Foo(456), new Foo(789) },
AcceptNullValue = false,
DefaultInstance = () => new Bar("Hello"),
InvalidValues =
{
{ typeof(ArgumentOutOfRangeException), new Foo(-123), new Foo(-456) },
{ typeof(ArgumentException), new Foo(666) }
}
};
}
The contract verifier generates the following unit tests:
Have a look at the MbUnit test project for more usage examples.

fully initialized class

I am using initiazling property in a class
and i want to run a validation method after it fully initialized.
i cant use the constructor for obvious reasons. is there a way to do that in some kind of Class initialized event ?
var t = new Foo
{
foo = "";
}
class Foo
{
public string foo {get; set;}
...
public bool validate {get ; set;}
private void validation()
{
if(foo == "")
validate = false;
if ...
}
}

(Note: for clarity, I renamed the property to Bar, in order to easily distinguish it from the type Foo)
If the Bar property must be valid upon construction, why are you not requiring it in the constructor? Why are you allowing the construction of invalid objects?
class Foo
{
public Foo(string bar) {
if(!IsValidBar(bar))
throw new ArgumentException("bar is not valid.", "bar");
this.Bar = bar;
}
public string Bar {get; set;}
private bool IsValidBar(string bar)
{
// blah blah
}
}
Alternatively, if you can construct an instance of Foo without the value of the Bar property, but you don't want to allow setting Bar to an invalid value, you can validate this in the setter:
class Foo
{
private string bar;
public string Bar
{
get { return bar; }
set
{
if(!IsValidBar(value))
throw new ArgumentException("bar is not valid.", "value");
bar = value;
}
}
private bool IsValidBar(string bar)
{
// blah blah
}
}

You can add verification logic to properties. Verify if class initialized after property assigned and raise static event if initialization completed. You can get reference to instance by casting event sender to Foo.
public string Foo
{ get { return _foo; }
set
{
_foo = value;
if (IsInitialized)
OnClassInitialized();
}
}
public static event EventHandler ClassInitialized;
private OnClassInitialized()
{
if (ClassInitialized != null)
ClassInitialized(this, EventArgs.Empty);
}
Usage:
Foo.ClassInitialized += (sender, e) =>
{
Foo foo = sender as Foo;
...
};

One approach is an interface designed for validation purposes; IValidation for instance. IValidation could then contain a Validate method. Classes which need to provide the behavior can now do so in a manageable way.
This prevents bloating within the constructor which IMHO is bad design.

You can use Aspect Oriented Programming like postsharp. http://www.postsharp.org/. But you lose on performance.

You can avoid using proprty initializers, and just move all that code to the constructor, using optional parameters if there are lots of properties. That way, you'll get kind-of property initializer constructor, and yet still be able to validate the class after the initialization is done. Something like this:
class Foo
{
public string Foo {get; set;}
public string Bar {get; set;}
public bool IsValid {get ; set;}
private void Validation()
{
if(foo == "")
IsValid = false;
if ...
}
public void Foo(string foo = string.Empty, string bar = string.Empty)
{
Foo = foo;
Bar = bar;
Validation();
}
}
.....
var t = new Foo (Foo = "SomeString");
The downside is that this is relatively new C# 4 syntax.
If you can't use c# 4, you could use the property accessors to enable validation, e.g. like:
public string Foo
{
get { return foo; }
set
{
foo = value;
Validation();
}
}
but this will evaluate the validity on each set, and might be slow if you set a lot of properties at once. You could also use the get accessor on combination with some lazy loading, something like this:
public bool IsValid
{
get
{
if (!isValidated)
Validation();
return isValid;
}
private set { isValid = value; }
}
public string Foo
{
get { return foo; }
set
{
foo = value;
isValidated := false;
}
}

How to share a variable between two classes?

How would you share the same object between two other objects? For instance, I'd like something in that flavor:
class A
{
private string foo_; // It could be any other class/struct too (Vector3, Matrix...)
public A (string shared)
{
this.foo_ = shared;
}
public void Bar()
{
this.foo_ = "changed";
}
}
...
// inside main
string str = "test";
A a = new A(str);
Console.WriteLine(str); // "test"
a.Bar();
Console.WriteLine(str); // I get "test" instead of "changed"... :(
Here, I don't want to give a ref to the Bar method. What I want to achieve is something that would look like that in C++:
class A
{
int* i;
public:
A(int* val);
};
A::A (int* val)
{
this->i = val;
}
I read there is some ref/out stuff, but I couldn't get what I'm asking here. I could only apply some changes in the methods scope where I was using ref/out arguments...
I also read we could use pointers, but is there no other way to do it?

This has nothing to do with sharing objects. You passed a reference to a string into the A constructor. That reference was copied into the private member foo_. Later, you called B(), which changed foo_ to "changed".
At no time did you modify str. str is a local variable in main. You never passed a reference to it.
If you had wanted to change str, you could have defined B as
public void Bar(ref string s)
{
this.foo_ = "changed";
s = this.foo_;
}
Consider:
public class C
{
public int Property {get;set;}
}
public class A
{
private C _c;
public A(C c){_c = c;}
public void ChangeC(int n) {_c.Property = n;}
}
public class B
{
private C _c;
public B(C c){_c = c;}
public void ChangeC(int n) {_c.Property = n;}
}
in main:
C myC = new C() {Property = 1;}
A myA = new A(myC);
B myB = new B(myC);
int i1 = myC.Property; // 1
myA.ChangeC(2);
int i2 = myC.Property; // 2
myB.ChangeC(3);
int i3 = myC.Property; // 3

Wrap your string inside a class. You need to do this because strings are immutable. Any attempt to change a string actually results in a new string.
class Foo {
class StringHolder {
public string Value { get; set; }
}
private StringHolder holder = new StringHolder();
public string Value {
get { return holder.Value; }
set { holder.Value = value; }
}
public Foo() { }
// this constructor creates a "linked" Foo
public Foo(Foo other) { this.holder = other.holder; }
}
// .. later ...
Foo a = new Foo { Value = "moose" };
Foo b = new Foo(a); // link b to a
b.Value = "elk";
// now a.Value also == "elk"
a.Value = "deer";
// now b.Value also == "deer"

I would split my answer to 2 parts:
1) If the variable is a reference type than it is already shared since you pass its reference to all interested objects. The only thing you should pay attention is that the reference type instances are mutable.
2) If the variable is a value type than you would have to use ref or out or some wrapper that is mutable and you can change the value inside the wrapper using a method or a property.
Hope that helps.

You need to pass the paramter as a reference to your method,
class A
{
private string foo_; // It could be any other class/struct too (Vector3, Matrix...)
public A(string shared)
{
this.foo_ = shared;
}
public void Bar(ref string myString)
{
myString = "changed";
}
}
static void Main()
{
string str = "test";
A a = new A(str);
Console.WriteLine(str); // "test"
a.Bar(ref str);
Console.WriteLine(str);
}

When a variable is a string, it is a reference.
Try to clone the string. http://msdn.microsoft.com/en-us/library/system.string.clone.aspx