I've researched this question quite a bit, and while I've found a lot about C# and parameterized properties (using an indexer is the only way), I haven't found an actual answer to my question.
First, what I'm trying to do:
I have an existing COM DLL written in VB6 and I'm trying to create a C# DLL that uses a similar interface. I say similar because the VB6 DLL is only used with late binding, so it doesn't have to have the same GUIDs for the calls (that is, it doesn't have to be "binary compatible"). This VB6 COM DLL uses parameterized properties in a few places, which I know aren't supported by C#.
When using a VB6 COM DLL with parameterized properties, the reference in C# will access them as methods in the form "get_PropName" and "set_PropName". However, I'm going in the opposite direction: I'm not trying to access the VB6 DLL in C#, I'm trying to make a C# COM DLL compatible with a VB6 DLL.
So, the question is: How do I make getter and setter methods in a C# COM DLL that appear as a single parameterized property when used by VB6?
For example, say the VB6 property is defined as follows:
Public Property Get MyProperty(Param1 As String, Param2 as String) As String
End Property
Public Property Let MyProperty(Param1 As String, Param2 As String, NewValue As String)
End Property
The equivalent in C# would be something like this:
public string get_MyProperty(string Param1, string Param2)
{
}
public void set_MyProperty(string Param1, string Param2, ref string NewValue)
{
}
So, how would I make those C# methods look like (and function like) a single parameterized property when used by VB6?
I tried creating two methods, one called "set_PropName" and the other "get_PropName", hoping it would figure out that they're supposed to be a single parameterized property when used by VB6, but that didn't work; they appeared as two different method calls from VB6.
I thought maybe some attributes needed to be applied to them in C# so that they'd be seen as a single parameterized property in COM and VB6, but I couldn't find any that seemed appropriate.
I also tried overloading the methods, removing "get_" and "set_", hoping it would see them as a single property, but that didn't work either. That one generated this error in VB6: "Property let procedure not defined and property get procedure did not return an object".
I'm almost positive that there should be a way of doing this, but I just can't seem to find it. Does anyone know how to do this?
Update:
I took Ben's advice and added an accessor class to see if this could solve my problem. However, now I'm running into another issue...
First, here's the COM interface I'm using:
[ComVisible(true),
Guid("94EC4909-5C60-4DF8-99AD-FEBC9208CE76"),
InterfaceType(ComInterfaceType.InterfaceIsDual)]
public interface ISystem
{
object get_RefInfo(string PropertyName, int index = 0, int subindex = 0);
void set_RefInfo(string PropertyName, int index = 0, int subindex = 0, object theValue);
RefInfoAccessor RefInfo { get; }
}
Here's the accessor class:
public class RefInfoAccessor
{
readonly ISystem mySys;
public RefInfoAccessor(ISystem sys)
{
this.mySys = sys;
}
public object this[string PropertyName, int index = 0, int subindex = 0]
{
get
{
return mySys.get_RefInfo(PropertyName, index, subindex);
}
set
{
mySys.set_RefInfo(PropertyName, index, subindex, value);
}
}
}
Here's the implementation:
[ComVisible(true)]
[ClassInterface(ClassInterfaceType.None)]
[Guid(MySystem.ClassId)]
[ProgId("MyApp.System")]
public class MySystem : ISystem
{
internal const string ClassId = "60A84737-8E96-4DF3-A052-7CEB855EBEC8";
public MySystem()
{
_RefInfo = new RefInfoAccessor(this);
}
public object get_RefInfo(string PropertyName, int index = 0, int subindex = 0)
{
// External code does the actual work
return "Test";
}
public void set_RefInfo(string PropertyName, int index = 0, int subindex = 0, object theValue)
{
// External code does the actual work
}
private RefInfoAccessor _RefInfo;
public RefInfoAccessor RefInfo
{
get
{
return _RefInfo;
}
}
}
Here's what I'm doing to test this in VB6, but I get an error:
Set sys = CreateObject("MyApp.System")
' The following statement gets this error:
' "Wrong number of arguments or invalid property assignment"
s = sys.RefInfo("MyTestProperty", 0, 0)
However, this works:
Set sys = CreateObject("MyApp.System")
Set obj = sys.RefInfo
s = obj("MyTestProperty", 0, 0)
It appears that it's trying to use the parameters on the property itself and getting an error because the property has no parameters. If I reference the RefInfo property in its own object variable, then it applies the indexer properties correctly.
Any ideas on how to arrange this so that it knows to apply the parameters to the accessor's indexer, rather than attempting to apply it to the property?
Also, how do I do a +1? This is my first question on StackOverflow :-)
Update #2:
Just to see how it would work, I also tried the default value approach. Here's how the accessor looks now:
public class RefInfoAccessor
{
readonly ISystem mySys;
private int _index;
private int _subindex;
private string _propertyName;
public RefInfoAccessor(ISystem sys, string propertyName, int index, int subindex)
{
this.mySys = sys;
this._index = index;
this._subindex = subindex;
this._propertyName = propertyName;
}
[DispId(0)]
public object Value
{
get
{
return mySys.get_RefInfo(_propertyName, _index, _subindex);
}
set
{
mySys.set_RefInfo(_propertyName, _index, _subindex, value);
}
}
}
This works great for a "get". However, when I try setting the value, .NET flips out with the following error:
Managed Debugging Assistant 'FatalExecutionEngineError' has detected a
problem in 'blahblah.exe'.
Additional information: The runtime has encountered a fatal error. The
address of the error was at 0x734a60f4, on thread 0x1694. The error
code is 0xc0000005. This error may be a bug in the CLR or in the
unsafe or non-verifiable portions of user code. Common sources of this
bug include user marshaling errors for COM-interop or PInvoke, which
may corrupt the stack.
I'm assuming the problem is that .NET tried setting the value to the method, rather to the default property of the returned object, or something similar. If I add ".Value" to the set line, it works fine.
Update #3: Success!
I finally got this working. There's a few things to look for, however.
First, the default value of the accessor must return a scaler, not an object, like so:
public class RefInfoAccessor
{
readonly ISystem mySys;
private int _index;
private int _subindex;
private string _propertyName;
public RefInfoAccessor(ISystem sys, string propertyName, int index, int subindex)
{
this.mySys = sys;
this._index = index;
this._subindex = subindex;
this._propertyName = propertyName;
}
[DispId(0)]
public string Value // <== Can't be "object"
{
get
{
return mySys.get_RefInfo(_propertyName, _index, _subindex).ToString();
}
set
{
mySys.set_RefInfo(_propertyName, _index, _subindex, value);
}
}
}
Second, when using the accessor, you need to make the return type an object:
public object RefInfo(string PropertyName, int index = 0, int subindex = 0)
{
return new RefInfoAccessor(this,PropertyName,index,subindex);
}
This will make C# happy, since the default value is a COM thing (dispid 0) and not a C# thing, so C# expects a RefInfoAccessor to be returned, not a string. Since RefInfoAccessor can be coerced into an object, no compiler error.
When used in VB6, the following will now all work:
s = sys.RefInfo("MyProperty", 0, 0)
Debug.Print s
sys.RefInfo("MyProperty", 0, 0) = "Test" ' This now works!
s = sys.RefInfo("MyProperty", 0)
Debug.Print s
Many thanks to Ben for his help on this!
C# can do indexed properties, but these must be implemented using a helper class which has an indexer. This method will work with early-bound VB but not with late-bound VB:
using System;
class MyClass {
protected string get_MyProperty(string Param1, string Param2)
{
return "foo: " + Param1 + "; bar: " + Param2;
}
protected void set_MyProperty(string Param1, string Param2, string NewValue)
{
// nop
}
// Helper class
public class MyPropertyAccessor {
readonly MyClass myclass;
internal MyPropertyAccessor(MyClass m){
myclass = m;
}
public string this [string param1, string param2]{
get {
return myclass.get_MyProperty(param1, param2);
}
set {
myclass.set_MyProperty(param1, param2, value);
}
}
}
public readonly MyPropertyAccessor MyProperty;
public MyClass(){
MyProperty = new MyPropertyAccessor(this);
}
}
public class Program
{
public static void Main()
{
Console.WriteLine("Hello World");
var mc = new MyClass();
Console.WriteLine(mc.MyProperty["a", "b"]);
}
}
There is a tutorial here:
https://msdn.microsoft.com/en-us/library/aa288464(v=vs.71).aspx
Late-bound VB Workaround
This is a workaround which takes advantage of two facts about VB. One is that in the array index operator is the same as the function call operator - round brackets (parens). The other is that VB will allow us to omit the name of the default property.
Read-only Properties
If the property is get-only, you don't need to bother with this. Just use a function, and this will behave the same as array access for late-bound code.
Read-Write properties
Using the two facts above, we can see that these are equivalent in VB
// VB Syntax: PropName could either be an indexed property or a function
varName = obj.PropName(index1).Value
obj.PropName(index1).Value = varName
// But if Value is the default property of obj.PropName(index1)
// this is equivalent:
varName = obj.PropName(index1)
obj.PropName(index1) = varName
This means that instead of doing the this:
//Property => Object with Indexer
// C# syntax
obj.PropName[index1];
We can do this:
// C# syntax
obj.PropName(index1).Value
So here is the example code, with a single parameter.
class HasIndexedProperty {
protected string get_PropertyName(int index1){
// replace with your own implementation
return string.Format("PropertyName: {0}", index1);
}
protected void set_PropertyName(int index1, string v){
// this is an example - put your implementation here
}
// This line provides the indexed property name as a function.
public string PropertyName(int index1){
return new HasIndexedProperty_PropertyName(this, index1);
}
public class HasIndexedProperty_PropertyName{
protected HasIndexedProperty _owner;
protected int _index1;
internal HasIndexedProperty_PropertyName(
HasIndexedProperty owner, int index1){
_owner = owner; _index1 = index1;
}
// This line makes the property Value the default
[DispId(0)]
public string Value{
get {
return _owner.get_PropertyName(_index1);
}
set {
_owner.set_PropertyName(_index1, value);
}
}
}
}
Limitations
The limitation is that to work, this depends on the call being made in a context where the result is coerced to a non-object type. For example
varName = obj.PropName(99)
Since the Set keyword was not used, VB knows that it must get the default property for use here.
Again, when passing to a function which takes for example a string, this will work. Internally VariantChangeType will be called to convert the object to the correct type, which if coercing to a non-object will access the default property.
The problem may occur when passing directly as a parameter to a function which takes a Variant as an argument. In this case the accessor object will be passed. As soon as the object is used in a non-object context (e.g. an assignment or conversion to string) the default property will be fetched. However this will be the value at the time it is converted, not the time it was originally accessed. This may or may not be an issue.
This issue can be worked around however by having the accessor object cache the value it returns to ensure it is the value as at the time the accessor was created.
This feature you seek is usually called "indexed properties". The flavor that VB6 uses is the flavor supported by COM interfaces.
This IDL fragment is similar to what VB6 would generate, and shows what's going on under the hood:
interface ISomething : IDispatch {
[id(0x68030001), propget]
HRESULT IndexedProp(
[in, out] BSTR* a, // Index 1
[in, out] BSTR* b, // Index 2
[out, retval] BSTR* );
[id(0x68030001), propput]
HRESULT IndexedProp(
[in, out] BSTR* a, // Index 1
[in, out] BSTR* b, // Index 2
[in, out] BSTR* );
[id(0x68030000), propget]
HRESULT PlainProp(
[out, retval] BSTR* );
[id(0x68030000), propput]
HRESULT PlainProp(
[in, out] BSTR* );
};
IndexedProp is a String property that takes two String parameters as indices. Contrast with PlainProp, which is of course a non-indexed conventional property.
Unfortunately, C# has very limited support for COM-style indexed properties.
C# 4.0 supports consuming COM objects (written elsewhere) that implement a COM interface with indexed properties. This was added to improve interoperability with COM Automation servers like Excel. However, it doesn't support declaring such an interface, or creating an object that implements such a COM interface even if legally declared elsewhere.
Ben's answer tells you how to create indexed properties in C# - or at least something that results in an equivalent syntax in C# code. If you just want the syntax flavor while writing C# code, that works great. But of course it's not a COM-style indexed property.
This is a limitation of the C# language, not the .NET platform. VB.NET does support COM indexed properties, because they had the mandate to replace VB6 and therefore needed to go the extra mile.
If you really want COM indexed properties, you could consider writing the COM version of your object in VB.NET, and have that object forward calls to your C# implementation. It sounds like a lot of work to me. Or port all your code to VB.NET. It really depends on how badly do you want it.
References
C# Team Blog: FAQ on new features in C# 4.0:
But this feature is available only for COM interop; you cannot create your own indexed properties in C# 4.0.
Why C# doesn't implement indexed properties?
Eric Lippert answers
COM-style Indexed Properties in VB.NET: Property with parameter
Say I have the following code:
class MyField : DynamicObject
{
public dynamic Value { get; private set; }
public override bool TryConvert(ConvertBinder binder, out object result)
{
result = binder.Type == Value.GetType() ? Value : null;
return result != null;
}
public MyField(dynamic v)
{
Value = v;
}
}
// ...
public static class Program
{
static void doSomething(ulong address) { /* ... */ }
public void Main(string[] args)
{
dynamic field = new MyField((ulong)12345);
doSomething(field); // fails as field is not a ulong.
doSomething((ulong)field); // succeeds as field can be casted to a ulong.
ulong field2 = field; // also succeeds
}
}
Is there a way to get the first call to doSomething to succeed? I'm writing a library to read a particular file format which uses serialized C-style structures; reading the file entails reading these saved structure definitions and then "populating" them with the data contained in the rest of the file. I have a "structure" DynamicObject class (to support dot-notation access) and a "field" DynamicObject class, which is primarily necessary to hold additional information on the contents of the field; although I could probably get rid of it, it would make certain other operations more difficult. What I'd like to do is just "pretend" MyField is a certain type (well, technically just any built-in primitive or array of primitives, including 2D arrays) and implicitly convert it to that type. However, the runtime fails to try to implicitly convert field to the type required by the underlying method signature if field doesn't match the type required.
In the vein of Greg's answer, I came up with a solution that makes the runtime happy. It's not exactly what I was originally looking for, but it seems like the best solution.
Since I already have a large if-else tree in my source wherein I take an array of bytes and interpret them as an actual value-type, and indeed my current source does use an underlying generic MyField<T>, so this works fine. I can't recall why I wanted MyField to be dynamic in the first place.
Anyway, this is a modified solution.
class MyField<T>
{
public dynamic Value { get; private set; }
public MyField(dynamic v) { Value = v; }
public static implicit operator T(MyField field)
{
return (T)field.Value;
}
}
I keep coming back to wanting the runtime to just figure out what it needs to cast MyField to at runtime but I guess it's not that big of a deal. If anyone comes up with something better, let me know. I'm going to keep this question open in the meantime.
You potentially might want to look into Generics. Coupled with an interface may make the dynamic usage far more viable.
public interface Helper <TInput, TOutput>
{
<TOutput> DoSomething(TInput input);
}
So when you use this interface with a class, you'll implement your type for both input and output. Which will give you quite a bit of flexibility, which should avoid those cast that you mentioned earlier. A small example, I mean you could obviously adjust it based on needs but I still don't understand what you're trying to really do.
When using an enum as a parameter in a method call, must I cast it to the parameters defined type?
The last 2 lines of code show the exact same method call, except one is cast to an int, the other is not. What should the outcome of these 2 lines be?
Note in my example that the file that includes the enum is a .cs file, the other (poorly written here) is an aspx.cs file. I don’t think it should matter at all, but maybe it does.
Thank you!
**fileOne.cs**
[Imported]
public class Foo
{
[Imported]
public class Bar
{
[Imported]
[PreserveCase]
public enum Bam
{
[PreserveCase]
Low = 10,
[PreserveCase]
Medium = 50,
[PreserveCase]
High = 100
}
}
…code…
[PreserveCase]
public static void someMethod(string aString, int aNumber) {}
…code…
}
**fileTwo.aspx.cs**
…code…
string someStuffForJscript = #”
function afunction(doesntMatter)
{{
Foo.Bar.someMethod(""This is a string."", (int)Foo.Bar.Bam.Medium);
Foo.Bar.someMethod(""This is a string."", Foo.Bar.Bam.Medium);
}}
*This is oversimplified code, but the concept remains. there are numerous reasons beyond what you see here as to why an enum is used.
When using an enum as a parameter in a method call, must I cast it to the parameters defined type?
From this page:
However, an explicit cast is necessary to convert from enum type to an integral type. For example, the following statement assigns the enumerator Sun to a variable of the type int by using a cast to convert from enum to int.
int x = (int)Days.Sun;
Can I locally reference a class in C#, instead of an instance of a class? The following code won't compile but, as an example, something like:
void someFunc()
{
var a = System.Math;
var b = a.Abs(4);
}
edit: In the real program it's not the System.Math class and I'm wanting to construct the class and return the constructed value. I didn't think originally that the context in which I wanted to use the class would be relevent, and probably it shouldn't be.
Anastasiosyal has an interesting idea with using a local Delegate to do it.
You can reference a class:
Type math = typeof(System.Math);
But you cannot call static methods on it using regular dot syntax:
// Wont compile:
math.Abs(5);
If you just want to shorten (and IMHO obfuscate) your code, you can reference classes with aliases via a using directive:
// Untested, but should work
namespace MyUnreadableCode {
using m = System.Math;
class Foo {
public static Int32 Absolut(Int32 a) {
return m.Abs(a);
}
}
}
You cannot assign a variable a value of a static class. The question is why would you want to do this, there are probably other ways that you could tackle your problem
e.g. you could use delegates to assign the operation you want to perform:
Func<int,int> operation = Math.Abs;
// then you could use it like so:
int processedValue = operation(-1);
In c# they're called Types. And you can assign them like:
Type a = typeof(SomeClass);
However, you would have to instantiate it to use it. What I believe you want is a static import like in java, but unfortunately, they do not exist in c#.
Short answer: Not like what you have above.
In C# 6.0 they introduced a static import feature, which can solve the problem.
using static System.Math;
class MyProgram
{
static void Main(string[] args)
{
var b = Abs(4); // No need to specify the name of Math class
}
}
As I understand you need to refer to the class with short name? try this (on top of the file, inside using statements section):
using m = System.Math;
later in your code:
m.Abs(...)
Makes sense?
No. It's not possible to treat a Type as a value where instance methods bind to static methods on the original type. In order to do this you would need to construct a new type which forwards it's method calls to the original type.
class MyMath {
public int Abs(int i) {
return Math.Abs(i);
}
}
var x = new MyMath();
Why is it that when I use dynamic with json.net I get a runtime binding exception then calling a method without casting but I can do assignments not problem
private static void Main()
{
dynamic json = JObject.Parse("{\"Test\":23}");
var t = json.Test;
int a = t; //Success
Prop = t; //Success
Func(t); //RuntimeBinderException
}
private static void Func(int i){}
private static int Prop { get; set; }
When I cast it to the correct type there are no errors but I would prefer to not have to do that. Am I doing something wrong, is this a problem in the json.net library or is a language restriction.
Edit:
This is to solve a problem where I don't have control over the methods signature and I don't want to cast it on every call.
This is because json.Test returns a JValue and JValue has a dynamic TryConvert. So if you do an implicit static conversion by pointing it to an int or cast to an int it will at runtime call that TryConvert and you have success. However if you use that dynamically typed variable in a method argument, the c# runtime looks for a method named Func with an argument that best matches 'JValue' it will not try to call 'TryConvert' for every permutation of a possible method (even if it's only one) thus you get the runtime binding error.
So the simplest solution is to just cast on every call or set a statically typed variable each time you want to pass a JValue as an argument.
There is actually a more general question and answer of this same issue too if you are looking for more info:
Pass a dynamic variable in a static parameter of a method in C# 4
private static void Func(dynamic i){}
will solve the issue.