Someone asked me the other day when they should use the parameter keyword out instead of ref. While I (I think) understand the difference between the ref and out keywords (that has been asked before) and the best explanation seems to be that ref == in and out, what are some (hypothetical or code) examples where I should always use out and not ref.
Since ref is more general, why do you ever want to use out? Is it just syntactic sugar?
You should use out unless you need ref.
It makes a big difference when the data needs to be marshalled e.g. to another process, which can be costly. So you want to avoid marshalling the initial value when the method doesn't make use of it.
Beyond that, it also shows the reader of the declaration or the call whether the initial value is relevant (and potentially preserved), or thrown away.
As a minor difference, an out parameter needs not be initialized.
Example for out:
string a, b;
person.GetBothNames(out a, out b);
where GetBothNames is a method to retrieve two values atomically, the method won't change behavior whatever a and b are. If the call goes to a server in Hawaii, copying the initial values from here to Hawaii is a waste of bandwidth. A similar snippet using ref:
string a = String.Empty, b = String.Empty;
person.GetBothNames(ref a, ref b);
could confuse readers, because it looks like the initial values of a and b are relevant (though the method name would indicate they are not).
Example for ref:
string name = textbox.Text;
bool didModify = validator.SuggestValidName(ref name);
Here the initial value is relevant to the method.
Use out to denote that the parameter is not being used, only set. This helps the caller understand that you're always initializing the parameter.
Also, ref and out are not just for value types. They also let you reset the object that a reference type is referencing from within a method.
You're correct in that, semantically, ref provides both "in" and "out" functionality, whereas out only provides "out" functionality. There are some things to consider:
out requires that the method accepting the parameter MUST, at some point before returning, assign a value to the variable. You find this pattern in some of the key/value data storage classes like Dictionary<K,V>, where you have functions like TryGetValue. This function takes an out parameter that holds what the value will be if retrieved. It wouldn't make sense for the caller to pass a value into this function, so out is used to guarantee that some value will be in the variable after the call, even if it isn't "real" data (in the case of TryGetValue where the key isn't present).
out and ref parameters are marshaled differently when dealing with interop code
Also, as an aside, it's important to note that while reference types and value types differ in the nature of their value, every variable in your application points to a location of memory that holds a value, even for reference types. It just happens that, with reference types, the value contained in that location of memory is another memory location. When you pass values to a function (or do any other variable assignment), the value of that variable is copied into the other variable. For value types, that means that the entire content of the type is copied. For reference types, that means that the memory location is copied. Either way, it does create a copy of the data contained in the variable. The only real relevance that this holds deals with assignment semantics; when assigning a variable or passing by value (the default), when a new assignment is made to the original (or new) variable, it does not affect the other variable. In the case of reference types, yes, changes made to the instance are available on both sides, but that's because the actual variable is just a pointer to another memory location; the content of the variable--the memory location--didn't actually change.
Passing with the ref keyword says that both the original variable and the function parameter will actually point to the same memory location. This, again, affects only assignment semantics. If a new value is assigned to one of the variables, then because the other points to the same memory location the new value will be reflected on the other side.
It depends on the compile context (See Example below).
out and ref both denote variable passing by reference, yet ref requires the variable to be initialized before being passed, which can be an important difference in the context of Marshaling (Interop: UmanagedToManagedTransition or vice versa)
MSDN warns:
Do not confuse the concept of passing by reference with the concept of reference types. The two concepts are not the same. A method parameter can be modified by ref regardless of whether it is a value type or a reference type. There is no boxing of a value type when it is passed by reference.
From the official MSDN Docs:
out:
The out keyword causes arguments to be passed by reference. This is similar to the ref keyword, except that ref requires that the variable be initialized before being passed
ref:
The ref keyword causes an argument to be passed by reference, not by value. The effect of passing by reference is that any change to the parameter in the method is reflected in the underlying argument variable in the calling method. The value of a reference parameter is always the same as the value of the underlying argument variable.
We can verify that the out and ref are indeed the same when the argument gets assigned:
CIL Example:
Consider the following example
static class outRefTest{
public static int myfunc(int x){x=0; return x; }
public static void myfuncOut(out int x){x=0;}
public static void myfuncRef(ref int x){x=0;}
public static void myfuncRefEmpty(ref int x){}
// Define other methods and classes here
}
in CIL, the instructions of myfuncOut and myfuncRef are identical as expected.
outRefTest.myfunc:
IL_0000: nop
IL_0001: ldc.i4.0
IL_0002: starg.s 00
IL_0004: ldarg.0
IL_0005: stloc.0
IL_0006: br.s IL_0008
IL_0008: ldloc.0
IL_0009: ret
outRefTest.myfuncOut:
IL_0000: nop
IL_0001: ldarg.0
IL_0002: ldc.i4.0
IL_0003: stind.i4
IL_0004: ret
outRefTest.myfuncRef:
IL_0000: nop
IL_0001: ldarg.0
IL_0002: ldc.i4.0
IL_0003: stind.i4
IL_0004: ret
outRefTest.myfuncRefEmpty:
IL_0000: nop
IL_0001: ret
nop: no operation, ldloc: load local, stloc: stack local, ldarg: load argument, bs.s: branch to target....
(See: List of CIL instructions )
Below are some notes which i pulled from this codeproject article on C# Out Vs Ref
It should be used only when we are expecting multiple outputs from a function or a method. A thought on structures can be also a good option for the same.
REF and OUT are keywords which dictate how data is passed from caller to callee and vice versa.
In REF data passes two way. From caller to callee and vice-versa.
In Out data passes only one way from callee to caller. In this case if Caller tried to send data to the callee it will be overlooked / rejected.
If you are a visual person then please see this yourtube video which demonstrates the difference practically https://www.youtube.com/watch?v=lYdcY5zulXA
Below image shows the differences more visually
You need to use ref if you plan to read and write to the parameter. You need to use out if you only plan to write. In effect, out is for when you'd need more than one return value, or when you don't want to use the normal return mechanism for output (but this should be rare).
There are language mechanics that assist these use cases. Ref parameters must have been initialized before they are passed to a method (putting emphasis on the fact that they are read-write), and out parameters cannot be read before they are assigned a value, and are guaranteed to have been written to at the end of the method (putting emphasis on the fact that they are write only). Contravening to these principles results in a compile-time error.
int x;
Foo(ref x); // error: x is uninitialized
void Bar(out int x) {} // error: x was not written to
For instance, int.TryParse returns a bool and accepts an out int parameter:
int value;
if (int.TryParse(numericString, out value))
{
/* numericString was parsed into value, now do stuff */
}
else
{
/* numericString couldn't be parsed */
}
This is a clear example of a situation where you need to output two values: the numeric result and whether the conversion was successful or not. The authors of the CLR decided to opt for out here since they don't care about what the int could have been before.
For ref, you can look at Interlocked.Increment:
int x = 4;
Interlocked.Increment(ref x);
Interlocked.Increment atomically increments the value of x. Since you need to read x to increment it, this is a situation where ref is more appropriate. You totally care about what x was before it was passed to Increment.
In the next version of C#, it will even be possible to declare variable in out parameters, adding even more emphasis on their output-only nature:
if (int.TryParse(numericString, out int value))
{
// 'value' exists and was declared in the `if` statement
}
else
{
// conversion didn't work, 'value' doesn't exist here
}
How to use in or out or ref in C#?
All keywords in C# have the same functionality but with some boundaries.
in arguments cannot be modified by the called method.
ref arguments may be modified.
ref must be initialized before being used by caller it can be read and updated in the method.
out arguments must be modified by the caller.
out arguments must be initialized in the method
Variables passed as in arguments must be initialized before being passed in a method call. However, the called method may not assign a value or modify the argument.
You can't use the in, ref, and out keywords for the following kinds of methods:
Async methods, which you define by using the async modifier.
Iterator methods, which include a yield return or yield break statement.
Still feel the need for a good summary, this is what I came up with.
Summary,
When we are inside the function, this is how we specify the variable data access control,
in = R
out = must W before R
ref = R+W
Explanation,
in
Function may only READ that variable.
out
Variable must not be initialised first because,
function MUST WRITE to it before READ.
ref
Function may READ/WRITE to that variable.
Why is it named as such?
Focusing on where data gets modified,
in
Data must only be set before entering (in) function.
out
Data must only be set before leaving (out) function.
ref
Data must be set before entering (in) function.
Data may be set before leaving (out) function.
out is more constraint version of ref.
In a method body, you need to assign to all out parameters before leaving the method.
Also an values assigned to an out parameter is ignored, whereas ref requires them to be assigned.
So out allows you to do:
int a, b, c = foo(out a, out b);
where ref would require a and b to be assigned.
How it sounds:
out = only initialize/fill a parameter (the parameter must be empty) return it out plain
ref = reference, standard parameter (maybe with value), but the function can modifiy it.
You can use the out contextual keyword in two contexts (each is a link to detailed information), as a parameter modifier or in generic type parameter declarations in interfaces and delegates. This topic discusses the parameter modifier, but you can see this other topic for information on the generic type parameter declarations.
The out keyword causes arguments to be passed by reference. This is like the ref keyword, except that ref requires that the variable be initialized before it is passed. To use an out parameter, both the method definition and the calling method must explicitly use the out keyword. For example:
C#
class OutExample
{
static void Method(out int i)
{
i = 44;
}
static void Main()
{
int value;
Method(out value);
// value is now 44
}
}
Although variables passed as out arguments do not have to be initialized before being passed, the called method is required to assign a value before the method returns.
Although the ref and out keywords cause different run-time behavior, they are not considered part of the method signature at compile time. Therefore, methods cannot be overloaded if the only difference is that one method takes a ref argument and the other takes an out argument. The following code, for example, will not compile:
C#
class CS0663_Example
{
// Compiler error CS0663: "Cannot define overloaded
// methods that differ only on ref and out".
public void SampleMethod(out int i) { }
public void SampleMethod(ref int i) { }
}
Overloading can be done, however, if one method takes a ref or out argument and the other uses neither, like this:
C#
class OutOverloadExample
{
public void SampleMethod(int i) { }
public void SampleMethod(out int i) { i = 5; }
}
Properties are not variables and therefore cannot be passed as out parameters.
For information about passing arrays, see Passing Arrays Using ref and out (C# Programming Guide).
You can't use the ref and out keywords for the following kinds of methods:
Async methods, which you define by using the async modifier.
Iterator methods, which include a yield return or yield break statement.
Example
Declaring an out method is useful when you want a method to return multiple values. The following example uses out to return three variables with a single method call. Note that the third argument is assigned to null. This enables methods to return values optionally.
C#
class OutReturnExample
{
static void Method(out int i, out string s1, out string s2)
{
i = 44;
s1 = "I've been returned";
s2 = null;
}
static void Main()
{
int value;
string str1, str2;
Method(out value, out str1, out str2);
// value is now 44
// str1 is now "I've been returned"
// str2 is (still) null;
}
}
Just to clarify on OP's comment that the use on ref and out is a "reference to a value type or struct declared outside the method", which has already been established in incorrect.
Consider the use of ref on a StringBuilder, which is a reference type:
private void Nullify(StringBuilder sb, string message)
{
sb.Append(message);
sb = null;
}
// -- snip --
StringBuilder sb = new StringBuilder();
string message = "Hi Guy";
Nullify(sb, message);
System.Console.WriteLine(sb.ToString());
// Output
// Hi Guy
As apposed to this:
private void Nullify(ref StringBuilder sb, string message)
{
sb.Append(message);
sb = null;
}
// -- snip --
StringBuilder sb = new StringBuilder();
string message = "Hi Guy";
Nullify(ref sb, message);
System.Console.WriteLine(sb.ToString());
// Output
// NullReferenceException
Basically both ref and out for passing object/value between methods
The out keyword causes arguments to be passed by reference. This is like the ref keyword, except that ref requires that the variable be initialized before it is passed.
out : Argument is not initialized and it must be initialized in the method
ref : Argument is already initialized and it can be read and updated in the method.
What is the use of “ref” for reference-types ?
You can change the given reference to a different instance.
Did you know?
Although the ref and out keywords cause different run-time behavior, they are not considered part of the method signature at compile time. Therefore, methods cannot be overloaded if the only difference is that one method takes a ref argument and the other takes an out argument.
You can't use the ref and out keywords for the following kinds of methods:
Async methods, which you define by using the async modifier.
Iterator methods, which include a yield return or yield break statement.
Properties are not variables and therefore cannot be passed as out parameters.
An argument passed as ref must be initialized before passing to the method whereas out parameter needs not to be initialized before passing to a method.
why do you ever want to use out?
To let others know that the variable will be initialized when it returns from the called method!
As mentioned above:
"for an out parameter, the calling method is required to assign a value before the method returns."
example:
Car car;
SetUpCar(out car);
car.drive(); // You know car is initialized.
Extra notes regarding C# 7:
In C# 7 there's no need to predeclare variables using out. So a code like this:
public void PrintCoordinates(Point p)
{
int x, y; // have to "predeclare"
p.GetCoordinates(out x, out y);
WriteLine($"({x}, {y})");
}
Can be written like this:
public void PrintCoordinates(Point p)
{
p.GetCoordinates(out int x, out int y);
WriteLine($"({x}, {y})");
}
Source: What's new in C# 7.
It should be noted that in is a valid keyword as of C# ver 7.2:
The in parameter modifier is available in C# 7.2 and later. Previous versions generate compiler error CS8107 ("Feature 'readonly references' is not available in C# 7.0. Please use language version 7.2 or greater.") To configure the compiler language version, see Select the C# language version.
...
The in keyword causes arguments to be passed by reference. It makes the formal parameter an alias for the argument, which must be a variable. In other words, any operation on the parameter is made on the argument. It is like the ref or out keywords, except that in arguments cannot be modified by the called method. Whereas ref arguments may be modified, out arguments must be modified by the called method, and those modifications are observable in the calling context.
Related
If we have a struct and a class they will be passed differently if you call a function with them as parameter.
struct MyStruct{}
class MyClass{}
// s is a value (struct) which passed by value
private void WriteStruct(MyStruct s)
{
Console.WriteLine(s);
}
// c is a reference to an class-instance which is passed by value
private void WriteClass(MyClass c)
{
Console.WriteLine(c);
}
This behaviour is described here, here and here.
Now what happens if we have a function which accepts object.
private void Write(object obj)
{
Console.WriteLine(obj);
}
I'm aware of boxing and assume that it'll just be boxed implicitly but I'm unsure in this case, mostly because ValueType inherits from Object (source) and also because I don't see why it wouldn't just be passed as is without any boxing since that obviously works (at least) when the signature also uses that struct.
Will calling this method with a struct (or any other value type) box the struct and actually pass a reference or will the struct itself be passed?
Ps. I don't know how you could test this because if you were to cast it to the struct in the function, a new struct would be created1 and the reference (if there was one) would be lost anyway. Thats why the following code doesn't change the value no matter how obj was passed.
private static void ChangeObj(object obj)
{
var s = (MyStruct)obj; // this will create a new struct
s.MyInt = 400;
} // s is now lost, the passed in value is unchanged
EDIT:
TheGeneral mentioned in the comments to head over to sharplab. I have no clue about IL but when looking at the following code, I can see some interesting differences.
MyStruct s = new MyStruct();
MyClass c = new MyClass();
WriteStruct(s);
WriteClass(c);
Write(s);
The WriteStruct(s) instruction contains an IL instruction call instance void Test::WriteStruct(valuetype MyStruct) (and some other stuff I don't get).
The WriteClass(c) instruction contains an IL instruction call instance void Test::WriteClass(class MyClass) (and some other stuff I don't get).
The Write(s) instruction (drumroll) contains an IL instruction box MyStruct (woo) followed by call instance void Test::Write(object)
I guess this means the value is indeed boxed.. Is there anything to add to this?
I'm guessing it has to be boxed since the runtime treats all objects which are explicitly marked as object as references, is that correct?
If so, why is there this class keyword in the signature of the IL-call instance instruction when calling WriteClass but not when calling Write? There's just object without any class or valuetype keyword. Is this too much of implementation details and IL-gibbrish (btw IL looks cool, might look into that more) to include in an answer here or is it possible to explain this in an easy way?
1 Talking about creating might be wrong here since the values are just copied. MSDN says in the boxing-unboxing-article: Copying the value from the instance into the value-type variable.
Is there a specific reason why C# 7 bring inlining out parameters but not ref?
The following is valid on C# 7:
int.TryParse("123", out _);
But this is invalid:
public void Foo(ref int x) { }
Foo(ref _); // error
I don't see a reason why the same logic can't be applied to ref parameters.
The reason is simple: because you're not allowed to pass an uninitialized variable into a ref parameter. This has always been the case, and the new syntactical sugar in C#7 doesn't change that.
Observe:
int i;
MyOutParameterMethod(out i); // allowed
int j;
MyRefParameterMethod(ref j); // compile error
The new feature in C#7 allows you to create a variable in the process of calling a method with an out parameter. It doesn't change the rules about uninitialized variables. The purpose of a ref parameter is to allow passing an already-initialized value into a method and (optionally) allow the original variable to be changed. The compiler semantics inside the method body treat ref parameters as initialized variables and out parameters as uninitialized variables. And it remains that way in C#7.
I have the following class:
[StructLayout(LayoutKind.Sequential)]
class Class
{
public int Field1;
public byte Field2;
public short? Field3;
public bool Field4;
}
How can I get the byte offset of Field4 starting from the start of the class data (or object header)?
To illustrate:
Class cls = new Class();
fixed(int* ptr1 = &cls.Field1) //first field
fixed(bool* ptr2 = &cls.Field4) //requested field
{
Console.WriteLine((byte*)ptr2-(byte*)ptr1);
}
The resulting offset is, in this case, 5, because the runtime actually moves Field3 to the end of the type (and pads it), probably because it its type is generic. I know there is Marshal.OffsetOf, but it returns unmanaged offset, not managed.
How can I retrieve this offset from a FieldInfo instance? Is there any .NET method used for that, or do I have to write my own, taking all the exceptions into account (type size, padding, explicit offsets, etc.)?
Offset of a field within a class or struct in .NET 4.7.2:
public static int GetFieldOffset(this FieldInfo fi) =>
GetFieldOffset(fi.FieldHandle);
public static int GetFieldOffset(RuntimeFieldHandle h) =>
Marshal.ReadInt32(h.Value + (4 + IntPtr.Size)) & 0xFFFFFF;
These return the byte offset of a field within a class or struct, relative to the layout of some respective managed instance at runtime. This works for all StructLayout modes, and for both value- and reference-types (including generics, reference-containing, or otherwise non-blittable). The offset value is zero-based relative to the beginning of the user-defined content or 'data body' of the struct or class only, and doesn't include any header, prefix, or other pad bytes.
Discussion
Since struct types have no header, the returned integer offset value can used directly via pointer arithmetic, and System.Runtime.CompilerServices.Unsafe if necessary (not shown here). Reference-type objects, on the other hand, have a header which has to be skipped-over in order to reference the desired field. This object header is usually a single IntPtr, which means IntPtr.Size needs to be added to the the offset value. It is also necessary to dereference the GC ("garbage collection") handle to obtain the object's address in the first place.
With these considerations, we can synthesize a tracking reference to the interior of a GC object at runtime by combining the field offset (obtained via the method shown above) with an instance of the class (e.g. an Object handle).
The following method, which is only meaningful for class (and not struct) types, demonstrates the technique. For simplicity, it uses ref-return and the System.Runtime.CompilerServices.Unsafe libary. Error checking, such as asserting fi.DeclaringType.IsSubclassOf(obj.GetType()) for example, is also elided for simplicity.
/// <summary>
/// Returns a managed reference ("interior pointer") to the value or instance of type 'U'
/// stored in the field indicated by 'fi' within managed object instance 'obj'
/// </summary>
public static unsafe ref U RefFieldValue<U>(Object obj, FieldInfo fi)
{
var pobj = Unsafe.As<Object, IntPtr>(ref obj);
pobj += IntPtr.Size + GetFieldOffset(fi.FieldHandle);
return ref Unsafe.AsRef<U>(pobj.ToPointer());
}
This method returns a managed "tracking" pointer into the interior of the garbage-collected object instance obj.[see comment] It can be used to arbitrarily read or write the field, so this one function replaces the traditional pair of separate getter/setter functions. Although the returned pointer cannot be stored in the GC heap and thus has a lifetime limited to the scope of the current stack frame (i.e., and below), it is very cheap to obtain at any time by simply calling the function again.
Note that this generic method is only parameterized with <U>, the type of the fetched pointed-at value, and not for the type ("<T>", perhaps) of the containing class (the same applies for the IL version below). It's because the bare-bones simplicity of this technique doesn't require it. We already know that the containing instance has to be a reference (class) type, so at runtime it will present via a reference handle to a GC object with object header, and those facts alone are sufficient here; nothing further needs to be known about putative type "T".
It's a matter of opinion whether adding vacuous <T, … >, which would allow us to indicate the where T: class constraint, would improve the look or feel of the example above. It certainly wouldn't hurt anything; I believe the JIT is smart enough to not generate additional generic method instantiations for generic arguments that have no effect. But since doing so seems chatty (other than for stating the constraint), I opted for the minimalism of strict necessity here.
In my own use, rather than passing a FieldInfo or its respective FieldHandle every time, what I actually retain are the various integer offset values for the fields of interest as returned from GetFieldOffset, since these are also invariant at runtime, once obtained. This eliminates the extra step (of calling GetFieldOffset) each time the pointer is fetched. In fact, since I am able to include IL code in my projects, here is the exact code that I use for the function above. As with the C# just shown, it trivially synthesizes a managed pointer from a containing GC-object obj, plus a (retained) integer offset offs within it.
// Returns a managed 'ByRef' pointer to the (struct or reference-type) instance of type U
// stored in the field at byte offset 'offs' within reference type instance 'obj'
.method public static !!U& RefFieldValue<U>(object obj, int32 offs) aggressiveinlining
{
ldarg obj
ldarg offs
sizeof object
add
add
ret
}
So even if you are not able to directly incorporate this IL, showing it here, I think, nicely illustrates the extremely low runtime overhead and alluring simplicity, in general, of this technique.
Example usage
class MyClass { public byte b_bar; public String s0, s1; public int iFoo; }
The first demonstration gets the integer offset of reference-typed field s1 within an instance of MyClass, and then uses it to get and set the field value.
var fi = typeof(MyClass).GetField("s1");
// note that we can get a field offset without actually
// having any instance of 'MyClass'
var offs = GetFieldOffset(fi);
// i.e., later...
var mc = new MyClass();
RefFieldValue<String>(mc, offs) = "moo-maa"; // field "setter"
// note: method call used as l-value, on the left-hand side of '=' assignment!
RefFieldValue<String>(mc, offs) += "!!"; // in-situ access
Console.WriteLine(mc.s1); // --> moo-maa!! (in the original)
// can be used as a non-ref "getter" for by-value access
var _ = RefFieldValue<String>(mc, offs) + "%%"; // 'mc.s1' not affected
If this seems a bit cluttered, you can dramatically clean it up by retaining the managed pointer as ref local variable. As you know, this type of pointer is automatically adjusted--with interior offset preserved--whenever the GC moves the containing object. This means that it will remain valid even as you continue accessing the field unawares. In exchange for allowing this capability, the CLR requires that the ref local variable itself not be allowed to escape its stack frame, which in this case is enforced by the C# compiler.
// demonstrate using 'RuntimeFieldHandle', and accessing a value-type
// field (int) this time
var h = typeof(MyClass).GetField(nameof(mc.iFoo)).FieldHandle;
// later... (still using 'mc' instance created above)
// acquire managed pointer to 'mc.iFoo'
ref int i = ref RefFieldValue<int>(mc, h);
i = 21; // directly affects 'mc.iFoo'
Console.WriteLine(mc.iFoo == 21); // --> true
i <<= 1; // operates directly on 'mc.iFoo'
Console.WriteLine(mc.iFoo == 42); // --> true
// any/all 'ref' uses of 'i' just affect 'mc.iFoo' directly:
Interlocked.CompareExchange(ref i, 34, 42); // 'mc.iFoo' (and 'i' also): 42 -> 34
Summary
The usage examples focused on using the technique with a class object, but as noted, the GetFieldOffset method shown here works perfectly fine with struct as well. Just be sure not to use the RefFieldValue method with value-types, since that code includes adjusting for an expected object header. For that simpler case, just use System.Runtime.CompilerServicesUnsafe.AddByteOffset for your address arithmetic instead.
Needless to say, this technique might seem a bit radical to some. I'll just note that it has worked flawlessly for me for many years, specifically on .NET Framework 4.7.2, and including 32- and 64-bit mode, debug vs. release, plus whichever various JIT optimization settings I've tried.
With some tricks around TypedReference.MakeTypedReference, it is possible to obtain the reference to the field, and to the start of the object's data, then just subtract. The method can be found in SharpUtils.
How can I change struct in external method ?
public void ChangeStruct (MyStruct myStruct) {
myStruct.field1 = 10;
return;
}
When I pass struct to ChangeStruct method after that method I would like myStruct to be changed.
You need to pass a reference to the struct instead of a copy using the ref keyword :
public void ChangeStruct (ref MyStruct myStruct)
{
myStruct.field1 = 10;
}
ChangeStruct(ref someStruct);
Your current code create a full bit-for-bit copy of the struct before entering the method and it's this copy that you are modifying, the ref keyword force the caller to pass a reference (managed pointer) to the structure instead of the copy.
You can use the ref keyword to observe changes to structs, but in the grand scheme, you will be in a world of less hurt if you simply use a class.
For an idea on when to use or not use structs, you might consult this link. A quick snippet that you may find helpful:
Do not define a structure unless the type has all of the following characteristics:
It logically represents a single value, similar to primitive types
(integer, double, and so on).
It has an instance size smaller than 16
bytes.
It is immutable.
It will not have to be boxed frequently.
Structs are value types, you must use the ref keyword to prevent copying. Using ref and out is not recommended, see When is using the C# ref keyword ever a good idea?.
When I develop in COM, I always see (void**) type conversion as below.
QueryInterface(/* [in] */ REFIID riid,/* [out] */ void** ppInterface)
What's exact meaning of it?
IMHO, it tells the compiler not to enforce type validation, since the type which is pointed by the ppInterface is not known to the client code at compile time.
Thanks~~~
Update 1
I understand it this way:
void* p implies AnyType* p
void ** pp implies pointer to AnyType*
Update 2
If void**pp means "pointer to void*", then what checks does the compiler do when it sees it?
A void ** is a pointer to a void *. This can be used to pass the address of a void * variable that will be used as an output parameter - eg:
void alloc_two(int n, void **a, void **b)
{
*a = malloc(n * 100);
*b = malloc(n * 200);
}
/* ... */
void *x;
void *y;
alloc_two(10, &x, &y);
The reason why COM uses void** with QueryInterface are somewhat special. (See below.)
Generally, void** simply means a pointer to void*, and it can be used for out parameters, ie. parameters that indicate a place where a function can return a value to. Your comment /* [out] */ indicates that the location pointed to by ppvInterface will be written to.
"Why can parameters with a pointer type be used as out parameters?", you ask? Remember that you can change two things with a pointer variable:
You can change the pointer itself, such that it points to another object. (ptr = ...)
You can modify the pointed-to object. (*ptr = ...)
Pointers are passed to a function by value, ie. the function gets its own local copy of the original pointer that was passed to it. This means you can change the pointer parameter inside the function (1) without affecting the original pointer, since only the local copy is modified. However, you can change the pointed-to object (2) and this will be visible outside of the function, because the copy has the same value as the original pointer and thus references the same object.
Now, about COM specifically:
A pointer to an interface (specified by riid) will be returned in the variable referenced by ppvInterface. QueryInterface achieves this via mechanism (2) mentioned above.
With void**, one * is required to allow mechanism (2); the other * reflects the fact that QueryInterface does not return a newly created object (IUnknown), but an already existing one: In order to avoid duplication of that object, a pointer to that object (IUnknown*) is returned.
If you're asking why ppvInterface has type void** and not IUnknown**, which would seem more reasonable type-safety-wise (since all interfaces must derive from IUnknown), then read the following argument taken from the book Essential COM by Don Box, p. 60 (chapter Type Coercion and IUnknown):
One additional subtlety related to QueryInterface concerns its second parameter, which is of type void **. It is very ironic that QueryInterface, the underpinning of the COM type system, has a fairly type-unsafe prototype in C++ [...]
IPug *pPug = 0;
hr = punk->QueryInterface(IID_IPug, (void**)&pPug);
Unfortunately, the following looks equally correct to the C++ compiler:
IPug *pPug = 0;
hr = punk->QueryInterface(IID_ICat, (void**)&pPug);
This more subtle variation also compiles correctly:
IPug *pPug = 0;
hr = punk->QueryInterface(IID_ICat, (void**)pPug);
Given that the rules of inheritance do not apply to pointers, this alternative definition of QueryInterface does not alleviate the problem:
HRESULT QueryInterface(REFIID riid, IUnknown** ppv);
The same limitation applies to references as to pointers as well. The following alternative definition is arguably more convenient for clients to use:
HRESULT QueryInterface(const IID& riid, void* ppv);
[...] Unfortunately, this solution does not reduce the number of errors [...] and, by eliminating the need for a cast, removes a visual indicator that C++ type safety might be in jeopardy. Given the desired semantics of QueryInterface, the argument types Microsoft chose are reasonable, if not type safe or elegant. [...]
It is just a pointer to void*.
Eg:
Something* foo;
Bar((void**)&foo);
// now foo points to something meaningful
Edit: A possible implementation in C#.
struct Foo { }
static Foo foo = new Foo();
unsafe static void Main(string[] args)
{
Foo* foo;
Bar((void**)&foo);
}
static unsafe void Bar(void** v)
{
fixed (Foo* f = &foo)
{
*v = f;
}
}
Passing by void * also ensures that the pointed to object cannot be deleted or tampered (accidentally).
"This implies that an object cannot be deleted using a pointer of type void* because there are no objects of type void."
It's a pointer to the interface pointer you request using this call. Obviously you can request all sorts of interfaces, so it has to be a void pointer. If the interface doesn't exist, the pointer is set to NULL.
edit: Detailed information to be found here: http://msdn.microsoft.com/en-us/library/ms682521(VS.85).aspx
It allows the API to specify that a pointer may be used as an [in-out] parameter in future, but for now, the pointer is unused. (NULL is usually the required value.)
When returning one of many possible types, with no common supertype (such as with QueryInterface), returning a void* is really the only option, and as this needs to be passed as an [out] parameter a pointer to that type (void**) is needed.
not to enforce type validation
Indeed, void* or void** are there to allow the use of different types of pointers, that can be downcasted to void* to fit in the function parameters type.
Pointer to pointer of unknown interface that can be provided.
Instead of using pointers to pointers, try using a reference to a pointer. It's a bit more C++ than using **.
e.g.
void Initialise(MyType &*pType)
{
pType = new MyType();
}