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Variable optimization with function

Suppose I call Print very often in my code.
Do I have to declare CurrentPosition as global or local variable?
I want to know which version is faster.
Option A:
int CurrentPosition = 0;
void Print(string key, int conc, int col, byte _color) {
if (col <= cMax ? col >= cMin : false) {
if (CurrentRows.TryGetValue(key + "#" + conc.ToString(), out CurrentPosition)) { //Row is in the vieport
grid.GetCell(CurrentPosition, col).Presenter.Background = new SolidColorBrush(Color.FromRgb(255, 255, 0));
}
}
}
Option B:
void Print(string key, int conc, int col, byte _color) {
int CurrentPosition = 0;
if (col <= cMax ? col >= cMin : false) {
if (CurrentRows.TryGetValue(key + "#" + conc.ToString(), out CurrentPosition)) { //Row is in the vieport
grid.GetCell(CurrentPosition, col).Presenter.Background = new SolidColorBrush(Color.FromRgb(255, 255, 0));
}
}
}

I propose to have a look under the hood to know how the C# compiler deals with these two options.
Let's say I have this simplistic class:
class Test
{
private int _currentPosition;
public int OptionA()
{
ImplOutParameter(out _currentPosition);
return _currentPosition;
}
public int OptionB()
{
int currentPosition;
ImplOutParameter(out currentPosition);
return currentPosition;
}
public void ImplOutParameter(out int position)
{
position = 1;
}
}
The two first methods are very similar to yours. The third one is here to test the out parameter implementation.
Let's compile this class in Release configuration. Using your favorite IL viewer, the method OptionA looks like:
It pushes the reference of the object on the stack twice to get the variable member reference and to call the ImplOutParameter method. Then it returns the value of the variable member.
The method OptionB looks like:
It pushes the reference of the object on the stack, pushes the address of the variable on the stack and call the ImplOutParameter method.Then it returns the value. It doesn't need to take time in order to allocate the variable.
Based on this analysis, I tend to say the OptionB implementation is a bit faster than the first one, but hey, this is very small. :)

If you're maintaining the state of CurrentPosition between calls and you're not passing the value into the method, then it needs to be outside. Otherwise, it starts fresh every time.
The best policy would be a third option of passing CurrentPosition into the method as a parameter, however, and maintain the state independent of the routine's code.
In terms of speed, allocation does take a small amount of time, but it's generally negligible. The "stack frame" will be lengthened regardless, and assigning a default value takes about as long as an assignment.
So, the most you'll save is a couple of cycles per call (i.e., nanonseconds), but yes, declaring it outside is going to be a hair faster.

If you want to know which is faster, test it. That's the only way to really be sure.
Always test performance if you're making a decision on what method to choose based on it. If it isn't worth testing, then you don't need the extra cycles.
That being said, the two methods are [i]functionally[/i] different, so your decision shouldn't be based on their speed. Option B sets CurrentPosition each time Print is called, whereas Option A sets CurrentPosition to 0 once and its value when Print is called is [i]not guaranteed to be 0[/i] so you should choose whichever option provides the correct function.
I can tell you that the speed different will likely be negligible, practically non-existent between the two.

A couple of questions about optimising code for speed in C#

If I have a method that is called many times, such as:
public int CalledManyTimes(int a, int b)
{
MyObject myObject = new myObject();
int c = a + b + myObject.GetSomeValue();
return c;
}
Is there a performance boost by putting MyObject myObject; outside of the method, so it's only declared once, or will the compiler do this automatically?
How exactly are structs passed around?
I'm passing in a Point struct to a method (Point contains only int x, int y), and that method is altering the value and returning a new Point(newX, newY); Is it better to alter the Point that was passed into the method and return that? Or can I create a Point point; outside the method as proposed in my first question and use that?

myObject appears to have no useful state; so: make that a static method - problem solved; no allocation, no virtual call:
public int CalledManyTimes(int a, int b)
{
int c = a + b + MyObject.GetSomeValue(); // static method
return c;
}
For anything else: profile.
Looking at your specific questions:
Is there a performance boost by putting MyObject myObject; outside of the method, so it's only declared once, or will the compiler do this automatically?
Initializing it zero times is even faster. However, if there is some state that isn't obvious in the question, then yes, I would expect it to be more efficient to reuse a single instance - however, that changes the semantic (in the original the state is not shared between iterations).
How exactly are structs passed around?
By default, they are copied on the stack as soon as you so much as glance in their direction. You can use ref to avoid the copy, which may be useful if the struct is massively overweight, or need to be updated (ideally with reassignment, rather than mutability).

Is modifying a value type from within a using statement undefined behavior?

This one's really an offshoot of this question, but I think it deserves its own answer.
According to section 15.13 of the ECMA-334 (on the using statement, below referred to as resource-acquisition):
Local variables declared in a
resource-acquisition are read-only, and shall include an initializer. A
compile-time error occurs if the
embedded statement attempts to modify
these local variables (via assignment
or the ++ and -- operators) or
pass them as ref or out
parameters.
This seems to explain why the code below is illegal.
struct Mutable : IDisposable
{
public int Field;
public void SetField(int value) { Field = value; }
public void Dispose() { }
}
using (var m = new Mutable())
{
// This results in a compiler error.
m.Field = 10;
}
But what about this?
using (var e = new Mutable())
{
// This is doing exactly the same thing, but it compiles and runs just fine.
e.SetField(10);
}
Is the above snippet undefined and/or illegal in C#? If it's legal, what is the relationship between this code and the excerpt from the spec above? If it's illegal, why does it work? Is there some subtle loophole that permits it, or is the fact that it works attributable only to mere luck (so that one shouldn't ever rely on the functionality of such seemingly harmless-looking code)?

I would read the standard in such a way that
using( var m = new Mutable() )
{
m = new Mutable();
}
is forbidden - with reason that seem obious.
Why for the struct Mutable it is not allowed beats me. Because for a class the code is legal and compiles fine...(object type i know..)
Also I do not see a reason why changing the contents of the value type does endanger the RA. Someone care to explain?
Maybe someone doing the syntx checking just misread the standard ;-)
Mario

I suspect the reason it compiles and runs is that SetField(int) is a function call, not an assignment or ref or out parameter call. The compiler has no way of knowing (in general) whether SetField(int) is going to mutate the variable or not.
This appears completely legal according to the spec.
And consider the alternatives. Static analysis to determine whether a given function call is going to mutate a value is clearly cost prohibitive in the C# compiler. The spec is designed to avoid that situation in all cases.
The other alternative would be for C# to not allow any method calls on value type variables declared in a using statement. That might not be a bad idea, since implementing IDisposable on a struct is just asking for trouble anyway. But when the C# language was first developed, I think they had high hopes for using structs in lots of interesting ways (as the GetEnumerator() example that you originally used demonstrates).

To sum it up
struct Mutable : IDisposable
{
public int Field;
public void SetField( int value ) { Field = value; }
public void Dispose() { }
}
class Program
{
protected static readonly Mutable xxx = new Mutable();
static void Main( string[] args )
{
//not allowed by compiler
//xxx.Field = 10;
xxx.SetField( 10 );
//prints out 0 !!!! <--- I do think that this is pretty bad
System.Console.Out.WriteLine( xxx.Field );
using ( var m = new Mutable() )
{
// This results in a compiler error.
//m.Field = 10;
m.SetField( 10 );
//This prints out 10 !!!
System.Console.Out.WriteLine( m.Field );
}
System.Console.In.ReadLine();
}
So in contrast to what I wrote above, I would recommend to NOT use a function to modify a struct within a using block. This seems wo work, but may stop to work in the future.
Mario

This behavior is undefined. In The C# Programming language at the end of the C# 4.0 spec section 7.6.4 (Member Access) Peter Sestoft states:
The two bulleted points stating "if the field is readonly...then
the result is a value" have a slightly surprising effect when the
field has a struct type, and that struct type has a mutable field (not
a recommended combination--see other annotations on this point).
He provides an example. I created my own example which displays more detail below.
Then, he goes on to say:
Somewhat strangely, if instead s were a local variable of struct type
declared in a using statement, which also has the effect of making s
immutable, then s.SetX() updates s.x as expected.
Here we see one of the authors acknowledge that this behavior is inconsistent. Per section 7.6.4, readonly fields are treated as values and do not change (copies change). Because section 8.13 tells us using statements treat resources as read-only:
the resource variable is read-only in the embedded statement,
resources in using statements should behave like readonly fields. Per the rules of 7.6.4 we should be dealing with a value not a variable. But surprisingly, the original value of the resource does change as demonstrated in this example:
//Sections relate to C# 4.0 spec
class Test
{
readonly S readonlyS = new S();
static void Main()
{
Test test = new Test();
test.readonlyS.SetX();//valid we are incrementing the value of a copy of readonlyS. This is per the rules defined in 7.6.4
Console.WriteLine(test.readonlyS.x);//outputs 0 because readonlyS is a value not a variable
//test.readonlyS.x = 0;//invalid
using (S s = new S())
{
s.SetX();//valid, changes the original value.
Console.WriteLine(s.x);//Surprisingly...outputs 2. Although S is supposed to be a readonly field...the behavior diverges.
//s.x = 0;//invalid
}
}
}
struct S : IDisposable
{
public int x;
public void SetX()
{
x = 2;
}
public void Dispose()
{
}
}
The situation is bizarre. Bottom line, avoid creating readonly mutable fields.

Dynamic Structs are failing

I have a problem using a class of made of structures.
Here's the basic definition:
using System;
struct Real
{
public double real;
public Real(double real)
{
this.real = real;
}
}
class Record
{
public Real r;
public Record(double r)
{
this.r = new Real(r);
}
public void Test(double origval, double newval)
{
if (this.r.real == newval)
Console.WriteLine("r = newval-test passed\n");
else if (this.r.real == origval)
Console.WriteLine("r = origval-test failed\n");
else
Console.WriteLine("r = neither-test failed\n");
}
}
When I create a non-dynamic (static?) Record, setting the Real works.
When I create a dynamic Record, setting the real doesn't work.
When I create a dynamic Record, replacing the real works.
And here's the test program
class Program
{
static void Main(string[] args)
{
double origval = 8.0;
double newval = 5.0;
// THIS WORKS - create fixed type Record, print, change value, print
Record record1 = new Record(origval);
record1.r.real = newval; // change value ***
record1.Test(origval, newval);
// THIS DOESN'T WORK. change value is not making any change!
dynamic dynrecord2 = new Record(origval);
dynrecord2.r.real = newval; // change value
dynrecord2.Test(origval, newval);
// THIS WORKS - create dynamic type Record, print, change value, print
dynamic dynrecord3 = new Record(origval);
dynamic r = dynrecord3.r; // copy out value
r.real = newval; // change copy
dynrecord3.r = r; // copy in modified value
dynrecord3.Test(origval, newval);
}
}
And here's the output:
r = newval-test passed
r = origval-test failed
r = newval-test passed
When I change the struct Real to class Real, all three cases work.
So what's going on?
Thanks,
Max

dynamic is really a fancy word for object as far as the core CLI is concerned, so you are mutating a boxed copy. This is prone to craziness. Mutating a struct in the first place is really, really prone to error. I would simply make the struct immutable - otherwise you are going to get this over and over.

I dug a little deeper into this problem. Here's an answer from Mads Torgersen of Microsoft.
From Mads:
This is a little unfortunate but by design. In
dynrecord2.r.real = newval; // change value
The value of dynrecord2.r gets boxed, which means copied into its own heap object. That copy is the one getting modified, not the original that you subsequently test.
This is a consequence of the very “local” way in which C# dynamic works. Think about a statement like the above – there are two fundamental ways that we could attack that:
1) Realize at compile time that something dynamic is going on, and essentially move the whole statement to be bound at runtime
2) Bind individual operations at runtime when their constituents are dynamic, returning something dynamic that may in turn cause things to be bound at runtime
In C# we went with the latter, which is nicely compositional, and makes it easy to describe dynamic in terms of the type system, but has some drawbacks – such as boxing of resulting value types for instance.
So what you are seeing is a result of this design choice.
I took another look at the MSIL. It essentially takes
dynrecord2.r.real = newval;
and turns it into:
Real temp = dynrecord2.r;
temp.real = newval;
If dynrecord2.r is a class, it just copies the handle so the change affects the internal field. If dynrecord2.r is a struct, a copy is made, and the change doesn't affect the original.
I'll leave it up to the reader to decide if this is a bug or a feature.
Max

Make your struct immutable and you won't have problems.
struct Real
{
private double real;
public double Real{get{return real;}}
public Real(double real)
{
this.real = real;
}
}
Mutable structs can be useful in native interop or some high performance scenarios, but then you better know what you're doing.

Can parameters be constant?

I'm looking for the C# equivalent of Java's final. Does it exist?
Does C# have anything like the following:
public Foo(final int bar);
In the above example, bar is a read only variable and cannot be changed by Foo(). Is there any way to do this in C#?
For instance, maybe I have a long method that will be working with x, y, and z coordinates of some object (ints). I want to be absolutely certain that the function doesn't alter these values in any way, thereby corrupting the data. Thus, I would like to declare them readonly.
public Foo(int x, int y, int z) {
// do stuff
x++; // oops. This corrupts the data. Can this be caught at compile time?
// do more stuff, assuming x is still the original value.
}

Unfortunately you cannot do this in C#.
The const keyword can only be used for local variables and fields.
The readonly keyword can only be used on fields.
NOTE: The Java language also supports having final parameters to a method. This functionality is non-existent in C#.
from http://www.25hoursaday.com/CsharpVsJava.html
EDIT (2019/08/13):
I'm throwing this in for visibility since this is accepted and highest on the list. It's now kind of possible with in parameters. See the answer below this one for details.

This is now possible in C# version 7.2:
You can use the in keyword in the method signature. MSDN documentation.
The in keyword should be added before specifying a method's argument.
Example, a valid method in C# 7.2:
public long Add(in long x, in long y)
{
return x + y;
}
While the following is not allowed:
public long Add(in long x, in long y)
{
x = 10; // It is not allowed to modify an in-argument.
return x + y;
}
Following error will be shown when trying to modify either x or y since they are marked with in:
Cannot assign to variable 'in long' because it is a readonly variable
Marking an argument with in means:
This method does not modify the value of the argument used as this parameter.

The answer: C# doesn't have the const functionality like C++.
I agree with Bennett Dill.
The const keyword is very useful. In the example, you used an int and people don't get your point. But, why if you parameter is an user huge and complex object that can't be changed inside that function? That's the use of const keyword: parameter can't change inside that method because [whatever reason here] that doesn't matters for that method. Const keyword is very powerful and I really miss it in C#.

Here's a short and sweet answer that will probably get a lot of down votes. I haven't read all of the posts and comments, so please forgive me if this has been previously suggested.
Why not take your parameters and pass them into an object that exposes them as immutable and then use that object in your method?
I realize this is probably a very obvious work around that has already been considered and the OP is trying to avoid doing this by asking this question, but I felt it should be here none-the-less...
Good luck :-)

I'll start with the int portion. int is a value type, and in .Net that means you really are dealing with a copy. It's a really weird design constraint to tell a method "You can have a copy of this value. It's your copy, not mine; I'll never see it again. But you can't change the copy." It's implicit in the method call that copying this value is okay, otherwise we couldn't have safely called the method. If the method needs the original, leave it to the implementer to make a copy to save it. Either give the method the value or do not give the method the value. Don't go all wishy-washy in between.
Let's move on to reference types. Now it gets a little confusing. Do you mean a constant reference, where the reference itself cannot be changed, or a completely locked, unchangeable object? If the former, references in .Net by default are passed by value. That is, you get a copy of the reference. So we have essentially the same situation as for value types. If the implementor will need the original reference they can keep it themselves.
That just leaves us with constant (locked/immutable) object. This might seem okay from a runtime perspective, but how is the compiler to enforce it? Since properties and methods can all have side effects, you'd essentially be limited to read-only field access. Such an object isn't likely to be very interesting.

Create an interface for your class that has only readonly property accessors. Then have your parameter be of that interface rather than the class itself. Example:
public interface IExample
{
int ReadonlyValue { get; }
}
public class Example : IExample
{
public int Value { get; set; }
public int ReadonlyValue { get { return this.Value; } }
}
public void Foo(IExample example)
{
// Now only has access to the get accessors for the properties
}
For structs, create a generic const wrapper.
public struct Const<T>
{
public T Value { get; private set; }
public Const(T value)
{
this.Value = value;
}
}
public Foo(Const<float> X, Const<float> Y, Const<float> Z)
{
// Can only read these values
}
Its worth noting though, that its strange that you want to do what you're asking to do regarding structs, as the writer of the method you should expect to know whats going on in that method. It won't affect the values passed in to modify them within the method, so your only concern is making sure you behave yourself in the method you're writing. There comes a point where vigilance and clean code are the key, over enforcing const and other such rules.

I know this might be little late.
But for people that are still searching other ways for this, there might be another way around this limitation of C# standard.
We could write wrapper class ReadOnly<T> where T : struct.
With implicit conversion to base type T.
But only explicit conversion to wrapper<T> class.
Which will enforce compiler errors if developer tries implicit set to value of ReadOnly<T> type.
As I will demonstrate two possible uses below.
USAGE 1 required caller definition to change. This usage will have only use in testing for correctness of your "TestCalled" functions code. While on release level/builds you shouldn't use it. Since in large scale mathematical operations might overkill in conversions, and make your code slow. I wouldn't use it, but for demonstration purpose only I have posted it.
USAGE 2 which I would suggest, has Debug vs Release use demonstrated in TestCalled2 function. Also there would be no conversion in TestCaller function when using this approach, but it requires a little more of coding of TestCaller2 definitions using compiler conditioning. You can notice compiler errors in debug configuration, while on release configuration all code in TestCalled2 function will compile successfully.
using System;
using System.Collections.Generic;
public class ReadOnly<VT>
where VT : struct
{
private VT value;
public ReadOnly(VT value)
{
this.value = value;
}
public static implicit operator VT(ReadOnly<VT> rvalue)
{
return rvalue.value;
}
public static explicit operator ReadOnly<VT>(VT rvalue)
{
return new ReadOnly<VT>(rvalue);
}
}
public static class TestFunctionArguments
{
static void TestCall()
{
long a = 0;
// CALL USAGE 1.
// explicite cast must exist in call to this function
// and clearly states it will be readonly inside TestCalled function.
TestCalled(a); // invalid call, we must explicit cast to ReadOnly<T>
TestCalled((ReadOnly<long>)a); // explicit cast to ReadOnly<T>
// CALL USAGE 2.
// Debug vs Release call has no difference - no compiler errors
TestCalled2(a);
}
// ARG USAGE 1.
static void TestCalled(ReadOnly<long> a)
{
// invalid operations, compiler errors
a = 10L;
a += 2L;
a -= 2L;
a *= 2L;
a /= 2L;
a++;
a--;
// valid operations
long l;
l = a + 2;
l = a - 2;
l = a * 2;
l = a / 2;
l = a ^ 2;
l = a | 2;
l = a & 2;
l = a << 2;
l = a >> 2;
l = ~a;
}
// ARG USAGE 2.
#if DEBUG
static void TestCalled2(long a2_writable)
{
ReadOnly<long> a = new ReadOnly<long>(a2_writable);
#else
static void TestCalled2(long a)
{
#endif
// invalid operations
// compiler will have errors in debug configuration
// compiler will compile in release
a = 10L;
a += 2L;
a -= 2L;
a *= 2L;
a /= 2L;
a++;
a--;
// valid operations
// compiler will compile in both, debug and release configurations
long l;
l = a + 2;
l = a - 2;
l = a * 2;
l = a / 2;
l = a ^ 2;
l = a | 2;
l = a & 2;
l = a << 2;
l = a >> 2;
l = ~a;
}
}

If you often run into trouble like this then you should consider "apps hungarian". The good kind, as opposed to the bad kind. While this doesn't normally tries to express constant-ness of a method parameter (that's just too unusual), there is certainly nothing that stops you from tacking an extra "c" before the identifier name.
To all those aching to slam the downvote button now, please read the opinions of these luminaries on the topic:
Eric Lippert
Larry Osterman
Joel Spolsky

If struct is passed into a method, unless it's passed by ref, it will not be changed by the method it's passed into. So in that sense, yes.
Can you create a parameter whose value can't be assigned within the method or whose properties cannot be set while within the method? No. You cannot prevent the value from being assigned within the method, but you can prevent it's properties from being set by creating an immutable type.
The question isn't whether the parameter or it's properties can be assigned to within the method. The question is what it will be when the method exits.
The only time any outside data is going to be altered is if you pass a class in and change one of it's properties, or if you pass a value by using the ref keyword. The situation you've outlined does neither.

The recommended (well, by me) is to use an interface that provides read only access to the members. Remembering that if the "real" member is a reference type, then only provide access to an interface supporting read operations for that type -- recursing down the entire object hierarchy.