Develop Reference

Powershell module: Dynamic mandatory hierarchical parameters

So what I really want is somewhat usable tab completion in a PS module.
ValidateSet seems to be the way to go here.
Unfortunately my data is dynamic, so I cannot annotate the parameter with all valid values upfront.
DynamicParameters/IDynamicParameters seems to be the solution for that problem.
Putting these things together (and reducing my failure to a simple test case) we end up with:
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Management.Automation;
using System.Text;
using System.Threading.Tasks;
namespace PSDummy
{
[Cmdlet(VerbsCommon.Get, "BookDetails")]
public class GetBookDetails : Cmdlet, IDynamicParameters
{
IDictionary<string, string[]> m_dummyData = new Dictionary<string, string[]> {
{"Terry Pratchett", new [] {"Small Gods", "Mort", "Eric"}},
{"Douglas Adams", new [] {"Hitchhiker's Guide", "The Meaning of Liff"}}
};
private RuntimeDefinedParameter m_authorParameter;
private RuntimeDefinedParameter m_bookParameter;
protected override void ProcessRecord()
{
// Do stuff here..
}
public object GetDynamicParameters()
{
var parameters = new RuntimeDefinedParameterDictionary();
m_authorParameter = CreateAuthorParameter();
m_bookParameter = CreateBookParameter();
parameters.Add(m_authorParameter.Name, m_authorParameter);
parameters.Add(m_bookParameter.Name, m_bookParameter);
return parameters;
}
private RuntimeDefinedParameter CreateAuthorParameter()
{
var p = new RuntimeDefinedParameter(
"Author",
typeof(string),
new Collection<Attribute>
{
new ParameterAttribute {
ParameterSetName = "BookStuff",
Position = 0,
Mandatory = true
},
new ValidateSetAttribute(m_dummyData.Keys.ToArray()),
new ValidateNotNullOrEmptyAttribute()
});
// Actually this is always mandatory, but sometimes I can fall back to a default
// value. How? p.Value = mydefault?
return p;
}
private RuntimeDefinedParameter CreateBookParameter()
{
// How to define a ValidateSet based on the parameter value for
// author?
var p = new RuntimeDefinedParameter(
"Book",
typeof(string),
new Collection<Attribute>
{
new ParameterAttribute {
ParameterSetName = "BookStuff",
Position = 1,
Mandatory = true
},
new ValidateSetAttribute(new string[1] { string.Empty }/* cannot fill this, because I cannot access the author */),
new ValidateNotNullOrEmptyAttribute()
});
return p;
}
}
}
Unfortunately this tiny snippet causes a lot of issues already. Ordered descending:
I fail to see how I can create a connection between the parameters. If you pick an author, you should only be able to pick a book that matches the author. So far GetDynamicParameters() always seems stateless though: I see no way to access the value of a different/earlier dynamic parameter. Tried keeping it in a field, tried searching MyInvocation - no luck. Is that even possible?
How do you define a default value for mandatory parameter? Doesn't fit the silly example, but let's say you can store your favorite author. From now on I want to default to that author, but having a pointer to an author is still mandatory. Either you gave me a default (and can still specify something else) or you need to be explicit.
Tab completion for strings with spaces seems weird/broken/limited - because it doesn't enclose the value with quotes (like cmd.exe would do, for example, if you type dir C:\Program <tab>). So tab completion actually breaks the invocation (if the issues above would be resolved, Get-BookDetails Ter<tab> would/will expand to Get-BookDetails Terry Pratchett which puts the last name in parameter position 1 aka 'book'.
Shouldn't be so hard, surely someone did something similar already?
Update: After another good day of tinkering and fooling around I don't see a way to make this work. The commandlet is stateless and will be instantiated over and over again. At the point in time when I can define dynamic parameters (GetDynamicParameters) I cannot access their (current) values/see what they'd be bound to - e.g. MyInvocation.BoundParameters is zero. I'll leave the question open, but it seems as if this just isn't supported. All the examples I see add a dynamic parameter based on the value of a static one - and that's not relevant here. Bugger.

I think this works. Unfortunately, it uses reflection to get at some of the cmdlet's private members for your first bullet. I got the idea from Garrett Serack. I'm not sure if I completely understood how to do the default author, so I made it so that the last valid author is stored in a static field so you don't need -Author the next time.
Here's the code:
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Management.Automation;
using System.Text;
using System.Threading.Tasks;
namespace PSDummy
{
internal class DynParamQuotedString {
/*
This works around the PowerShell bug where ValidateSet values aren't quoted when necessary, and
adding the quotes breaks it. Example:
ValidateSet valid values = 'Test string' (The quotes are part of the string)
PowerShell parameter binding would interperet that as [Test string] (no single quotes), which wouldn't match
the valid value (which has the quotes). If you make the parameter a DynParamQuotedString, though,
the parameter binder will coerce [Test string] into an instance of DynParamQuotedString, and the binder will
call ToString() on the object, which will add the quotes back in.
*/
internal static string DefaultQuoteCharacter = "'";
public DynParamQuotedString(string quotedString) : this(quotedString, DefaultQuoteCharacter) {}
public DynParamQuotedString(string quotedString, string quoteCharacter) {
OriginalString = quotedString;
_quoteCharacter = quoteCharacter;
}
public string OriginalString { get; set; }
string _quoteCharacter;
public override string ToString() {
// I'm sure this is missing some other characters that need to be escaped. Feel free to add more:
if (System.Text.RegularExpressions.Regex.IsMatch(OriginalString, #"\s|\(|\)|""|'")) {
return string.Format("{1}{0}{1}", OriginalString.Replace(_quoteCharacter, string.Format("{0}{0}", _quoteCharacter)), _quoteCharacter);
}
else {
return OriginalString;
}
}
public static string[] GetQuotedStrings(IEnumerable<string> values) {
var returnList = new List<string>();
foreach (string currentValue in values) {
returnList.Add((new DynParamQuotedString(currentValue)).ToString());
}
return returnList.ToArray();
}
}
[Cmdlet(VerbsCommon.Get, "BookDetails")]
public class GetBookDetails : PSCmdlet, IDynamicParameters
{
IDictionary<string, string[]> m_dummyData = new Dictionary<string, string[]>(StringComparer.OrdinalIgnoreCase) {
{"Terry Pratchett", new [] {"Small Gods", "Mort", "Eric"}},
{"Douglas Adams", new [] {"Hitchhiker's Guide", "The Meaning of Liff"}},
{"An 'Author' (notice the ')", new [] {"A \"book\"", "Another 'book'","NoSpace(ButCharacterThatShouldBeEscaped)", "NoSpace'Quoted'", "NoSpace\"Quoted\""}} // Test value I added
};
protected override void ProcessRecord()
{
WriteObject(string.Format("Author = {0}", _author));
WriteObject(string.Format("Book = {0}", ((DynParamQuotedString) MyInvocation.BoundParameters["Book"]).OriginalString));
}
// Making this static means it should keep track of the last author used
static string _author;
public object GetDynamicParameters()
{
// Get 'Author' if found, otherwise get first unnamed value
string author = GetUnboundValue("Author", 0) as string;
if (!string.IsNullOrEmpty(author)) {
_author = author.Trim('\'').Replace(
string.Format("{0}{0}", DynParamQuotedString.DefaultQuoteCharacter),
DynParamQuotedString.DefaultQuoteCharacter
);
}
var parameters = new RuntimeDefinedParameterDictionary();
bool isAuthorParamMandatory = true;
if (!string.IsNullOrEmpty(_author) && m_dummyData.ContainsKey(_author)) {
isAuthorParamMandatory = false;
var m_bookParameter = new RuntimeDefinedParameter(
"Book",
typeof(DynParamQuotedString),
new Collection<Attribute>
{
new ParameterAttribute {
ParameterSetName = "BookStuff",
Position = 1,
Mandatory = true
},
new ValidateSetAttribute(DynParamQuotedString.GetQuotedStrings(m_dummyData[_author])),
new ValidateNotNullOrEmptyAttribute()
}
);
parameters.Add(m_bookParameter.Name, m_bookParameter);
}
// Create author parameter. Parameter isn't mandatory if _author
// has a valid author in it
var m_authorParameter = new RuntimeDefinedParameter(
"Author",
typeof(DynParamQuotedString),
new Collection<Attribute>
{
new ParameterAttribute {
ParameterSetName = "BookStuff",
Position = 0,
Mandatory = isAuthorParamMandatory
},
new ValidateSetAttribute(DynParamQuotedString.GetQuotedStrings(m_dummyData.Keys.ToArray())),
new ValidateNotNullOrEmptyAttribute()
}
);
parameters.Add(m_authorParameter.Name, m_authorParameter);
return parameters;
}
/*
TryGetProperty() and GetUnboundValue() are from here: https://gist.github.com/fearthecowboy/1936f841d3a81710ae87
Source created a dictionary for all unbound values; I had issues getting ValidateSet on Author parameter to work
if I used that directly for some reason, but changing it into a function to get a specific parameter seems to work
*/
object TryGetProperty(object instance, string fieldName) {
var bindingFlags = System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.Static | System.Reflection.BindingFlags.Public;
// any access of a null object returns null.
if (instance == null || string.IsNullOrEmpty(fieldName)) {
return null;
}
var propertyInfo = instance.GetType().GetProperty(fieldName, bindingFlags);
if (propertyInfo != null) {
try {
return propertyInfo.GetValue(instance, null);
}
catch {
}
}
// maybe it's a field
var fieldInfo = instance.GetType().GetField(fieldName, bindingFlags);
if (fieldInfo!= null) {
try {
return fieldInfo.GetValue(instance);
}
catch {
}
}
// no match, return null.
return null;
}
object GetUnboundValue(string paramName) {
return GetUnboundValue(paramName, -1);
}
object GetUnboundValue(string paramName, int unnamedPosition) {
// If paramName isn't found, value at unnamedPosition will be returned instead
var context = TryGetProperty(this, "Context");
var processor = TryGetProperty(context, "CurrentCommandProcessor");
var parameterBinder = TryGetProperty(processor, "CmdletParameterBinderController");
var args = TryGetProperty(parameterBinder, "UnboundArguments") as System.Collections.IEnumerable;
if (args != null) {
var currentParameterName = string.Empty;
object unnamedValue = null;
int i = 0;
foreach (var arg in args) {
var isParameterName = TryGetProperty(arg, "ParameterNameSpecified");
if (isParameterName != null && true.Equals(isParameterName)) {
string parameterName = TryGetProperty(arg, "ParameterName") as string;
currentParameterName = parameterName;
continue;
}
// Treat as a value:
var parameterValue = TryGetProperty(arg, "ArgumentValue");
if (currentParameterName != string.Empty) {
// Found currentParameterName's value. If it matches paramName, return
// it
if (currentParameterName.Equals(paramName, StringComparison.OrdinalIgnoreCase)) {
return parameterValue;
}
}
else if (i++ == unnamedPosition) {
unnamedValue = parameterValue; // Save this for later in case paramName isn't found
}
// Found a value, so currentParameterName needs to be cleared
currentParameterName = string.Empty;
}
if (unnamedValue != null) {
return unnamedValue;
}
}
return null;
}
}
}

How do I pass a string to a function requiring an Object?

I am using Chello (the c# wrapper for the Trello API). I need to pass the argument "createCard" as per the documentation here: https://trello.com/docs/api/card/index.html
And this is the function I am using from Chello:
public IEnumerable<CardUpdateAction> ForCard(string cardId, object args)
{
string queryString = BuildQueryString(args);
return GetRequest<List<CardUpdateAction>>("/cards/{0}/actions?{1}", cardId, queryString);
}
I have tried calling this in this way:
List<CardUpdateAction> cua = chello.CardUpdates.ForCard("5264d37736695b2821001d7a","createCard").ToList();
but I get the error: Parameter Count Mismatch
on this function:
protected static string BuildQueryString(object args)
{
string queryString = String.Empty;
if (args != null)
{
StringBuilder sb = new StringBuilder();
foreach (var prop in args.GetType().GetProperties())
{
sb.AppendFormat("{0}={1}&", prop.Name, prop.GetValue(args, null));
}
if (sb.Length > 0) sb.Remove(sb.Length - 1, 1);
queryString = sb.ToString();
}
return queryString;
}

The problem is the fact that your API you are using expects you to pass in a class that has public properties equal to the tags you want to use.
This is very easy to do using Anonymous Types (I am doing a slightly different example to help illustrate a point)
//This will cause BuildQueryString to return "actions=createCard&action_fields=data,type,date"
var options = new { actions = "createCard", action_fields = "data,type,date" };
List<CardUpdateAction> cua = chello.CardUpdates.ForCard("5264d37736695b2821001d7a",options).ToList();

string is an object. Every type in .NET platform inherits from Object. This is called Unified Type System.
On the other hand, we have the Liskov Substitution Principle, which put simply, says that if B is a subtype of A (B is A), then you should be able to use B, wherever A is used.
Based on these reasons, you can pass string to any method that accepts an object as an argument.
You can test it:
public void DoSomething(object args)
{
}
public void Main()
{
DoSomething("some string argument, instead of the object");
}
It works just fine. No error.

How to refer to an identifier without writing it into a string literal in C#?

I often want to do this:
public void Foo(Bar arg)
{
throw new ArgumentException("Argument is incompatible with " + name(Foo));
}
Because if I change the name of Foo the IDE will refactor my error message too, what won't happen if I put the name of the method (or any other kind of member identifier) inside a string literal. The only way I know of implementing "name" is by using reflection, but I think the performance loss outweighs the mantainability gain and it won't cover all kinds of identifiers.
The value of the expression between parenthesis could be computed at compile time (like typeof) and optimized to become one string literal by changing the language specification. Do you think this is a worthy feature?
PS: The first example made it look like the question is related only to exceptions, but it is not. Think of every situation you may want to reference a type member identifier. You'll have to do it through a string literal, right?
Another example:
[RuntimeAcessibleDocumentation(Description="The class " + name(Baz) +
" does its job. See method " + name(DoItsJob) + " for more info.")]
public class Baz
{
[RuntimeAcessibleDocumentation(Description="This method will just pretend " +
"doing its job if the argument " + name(DoItsJob.Arguments.justPretend) +
" is true.")]
public void DoItsJob(bool justPretend)
{
if (justPretend)
Logger.log(name(justPretend) + "was true. Nothing done.");
}
}
UPDATE: this question was posted before C# 6, but may still be relevant for those who are using previous versions of the language. If you are using C# 6 check out the nameof operator, which does pretty much the same thing as the name operator in the examples above.

well, you could cheat and use something like:
public static string CallerName([CallerMemberName]string callerName = null)
{
return callerName;
}
and:
public void Foo(Bar arg)
{
throw new ArgumentException("Argument is incompatible with " + CallerName());
}
Here, all the work is done by the compiler (at compile-time), so if you rename the method it will immediately return the correct thing.

If you simply want the current method name: MethodBase.GetCurrentMethod().Name
If it's a type typeof(Foo).Name
If you want the name of a variable/parameter/field/property, with a little Expression tree
public static string GetFieldName<T>(Expression<Func<T>> exp)
{
var body = exp.Body as MemberExpression;
if (body == null)
{
throw new ArgumentException();
}
return body.Member.Name;
}
string str = "Hello World";
string variableName = GetFieldName(() => str);
For method names it's a little more tricky:
public static readonly MethodInfo CreateDelegate = typeof(Delegate).GetMethod("CreateDelegate", BindingFlags.Static | BindingFlags.Public, null, new[] { typeof(Type), typeof(object), typeof(MethodInfo) }, null);
public static string GetMethodName<T>(Expression<Func<T>> exp)
{
var body = exp.Body as UnaryExpression;
if (body == null || body.NodeType != ExpressionType.Convert)
{
throw new ArgumentException();
}
var call = body.Operand as MethodCallExpression;
if (call == null)
{
throw new ArgumentException();
}
if (call.Method != CreateDelegate)
{
throw new ArgumentException();
}
var method = call.Arguments[2] as ConstantExpression;
if (method == null)
{
throw new ArgumentException();
}
MethodInfo method2 = (MethodInfo)method.Value;
return method2.Name;
}
and when you call them you have to specify the type of a compatible delegate (Action, Action<...>, Func<...> ...)
string str5 = GetMethodName<Action>(() => Main);
string str6 = GetMethodName<Func<int>>(() => Method1);
string str7 = GetMethodName<Func<int, int>>(() => Method2);
or more simply, without using expressions :-)
public static string GetMethodName(Delegate del)
{
return del.Method.Name;
}
string str8 = GetMethodName((Action)Main);
string str9 = GetMethodName((Func<int>)Method1);
string str10 = GetMethodName((Func<int, int>)Method2);

As has been covered, using this approach for exceptions seems unnecessary due to the method name being in the call stack on the exception.
In relation to the other example in the question of logging the parameter value, it seems PostSharp would be a good candidate here, and probably would allow lots of new features of this kind that you're interested in.
Have a look at this page on PostSharp which came up when I searched for how to use PostSharp to log parameter values (which it covers). An excerpt taken from that page:
You can get a lot of useful information with an aspect, but there are three popular categories:
Code information: function name, class name, parameter values, etc. This can help you to reduce guessing in pinning down logic flaws or edge-case scenarios
Performance information: keep track of how much time a method is taking
Exceptions: catch select/all exceptions and log information about them

The original question is named "How to refer to an identifier without writing it into a string literal in C#?" This answer does not answer that question, instead, it answers the question "How to refer to an identifier by writing its name into a string literal using a preprocessor?"
Here is a very simple "proof of concept" C# preprocessor program:
using System;
using System.IO;
namespace StackOverflowPreprocessor
{
/// <summary>
/// This is a C# preprocessor program to demonstrate how you can use a preprocessor to modify the
/// C# source code in a program so it gets self-referential strings placed in it.
/// </summary>
public class PreprocessorProgram
{
/// <summary>
/// The Main() method is where it all starts, of course.
/// </summary>
/// <param name="args">must be one argument, the full name of the .csproj file</param>
/// <returns>0 = OK, 1 = error (error message has been written to console)</returns>
static int Main(string[] args)
{
try
{
// Check the argument
if (args.Length != 1)
{
DisplayError("There must be exactly one argument.");
return 1;
}
// Check the .csproj file exists
if (!File.Exists(args[0]))
{
DisplayError("File '" + args[0] + "' does not exist.");
return 1;
}
// Loop to process each C# source file in same folder as .csproj file. Alternative
// technique (used in my real preprocessor program) is to read the .csproj file as an
// XML document and process the <Compile> elements.
DirectoryInfo directoryInfo = new DirectoryInfo(Path.GetDirectoryName(args[0]));
foreach (FileInfo fileInfo in directoryInfo.GetFiles("*.cs"))
{
if (!ProcessOneFile(fileInfo.FullName))
return 1;
}
}
catch (Exception e)
{
DisplayError("Exception while processing .csproj file '" + args[0] + "'.", e);
return 1;
}
Console.WriteLine("Preprocessor normal completion.");
return 0; // All OK
}
/// <summary>
/// Method to do very simple preprocessing of a single C# source file. This is just "proof of
/// concept" - in my real preprocessor program I use regex and test for many different things
/// that I recognize and process in one way or another.
/// </summary>
private static bool ProcessOneFile(string fileName)
{
bool fileModified = false;
string lastMethodName = "*unknown*";
int i = -1, j = -1;
try
{
string[] sourceLines = File.ReadAllLines(fileName);
for (int lineNumber = 0; lineNumber < sourceLines.Length - 1; lineNumber++)
{
string sourceLine = sourceLines[lineNumber];
if (sourceLine.Trim() == "//?GrabMethodName")
{
string nextLine = sourceLines[++lineNumber];
j = nextLine.IndexOf('(');
if (j != -1)
i = nextLine.LastIndexOf(' ', j);
if (j != -1 && i != -1 && i < j)
lastMethodName = nextLine.Substring(i + 1, j - i - 1);
else
{
DisplayError("Unable to find method name in line " + (lineNumber + 1) +
" of file '" + fileName + "'.");
return false;
}
}
else if (sourceLine.Trim() == "//?DumpNameInStringAssignment")
{
string nextLine = sourceLines[++lineNumber];
i = nextLine.IndexOf('\"');
if (i != -1 && i != nextLine.Length - 1)
{
j = nextLine.LastIndexOf('\"');
if (i != j)
{
sourceLines[lineNumber] =
nextLine.Remove(i + 1) + lastMethodName + nextLine.Substring(j);
fileModified = true;
}
}
}
}
if (fileModified)
File.WriteAllLines(fileName, sourceLines);
}
catch (Exception e)
{
DisplayError("Exception while processing C# file '" + fileName + "'.", e);
return false;
}
return true;
}
/// <summary>
/// Method to display an error message on the console.
/// </summary>
private static void DisplayError(string errorText)
{
Console.WriteLine("Preprocessor: " + errorText);
}
/// <summary>
/// Method to display an error message on the console.
/// </summary>
internal static void DisplayError(string errorText, Exception exceptionObject)
{
Console.WriteLine("Preprocessor: " + errorText + " - " + exceptionObject.Message);
}
}
}
And here's a test file, based on the first half of the original question:
using System;
namespace StackOverflowDemo
{
public class DemoProgram
{
public class Bar
{}
static void Main(string[] args)
{}
//?GrabMethodName
public void Foo(Bar arg)
{
//?DumpNameInStringAssignment
string methodName = "??"; // Will be changed as necessary by preprocessor
throw new ArgumentException("Argument is incompatible with " + methodName);
}
}
}
To make the running of the preprocessor program a part of the build process you modify the .csproj file in two places. Insert this line in the first section:
<UseHostCompilerIfAvailable>false</UseHostCompilerIfAvailable>
(This is optional - see here https://stackoverflow.com/a/12163384/253938 for more information.)
And at the end of the .csproj file replace some lines that are commented-out with these lines:
<Target Name="BeforeBuild">
<Exec WorkingDirectory="D:\Merlinia\Trunk-Debug\Common\Build Tools\Merlinia Preprocessor\VS2012 projects\StackOverflowPreprocessor\bin" Command="StackOverflowPreprocessor.exe "$(MSBuildProjectFullPath)"" />
</Target>
Now when you recompile the test program the line that says
string methodName = "??"; // Will be changed as necessary by preprocessor
will be magically converted to say
string methodName = "Foo"; // Will be changed as necessary by preprocessor
OK?

Version 6 of C# has introduced the nameof operator which works like the name operator described in the examples of the question, but with some restrictions. Here are some examples and excerpts from the C# FAQ blog:
(if x == null) throw new ArgumentNullException(nameof(x));
You can put more elaborate dotted names in a nameof expression, but that’s just to tell the compiler where to look: only the final identifier will be used:
WriteLine(nameof(person.Address.ZipCode)); // prints "ZipCode"
Note: there are small design changes to nameof since the Preview was built. In the preview, dotted expressions like in the last example, where person is a variable in scope, are not allowed. Instead you have to dot in through the type.

How do I get the current line number?

Here is an example of what I want to do:
MessageBox.Show("Error line number " + CurrentLineNumber);
In the code above the CurrentLineNumber, should be the line number in the source code of this piece of code.
How can I do that?

In .NET 4.5 / C# 5, you can get the compiler to do this work for you, by writing a utility method that uses the new caller attributes:
using System.Runtime.CompilerServices;
static void SomeMethodSomewhere()
{
ShowMessage("Boo");
}
...
static void ShowMessage(string message,
[CallerLineNumber] int lineNumber = 0,
[CallerMemberName] string caller = null)
{
MessageBox.Show(message + " at line " + lineNumber + " (" + caller + ")");
}
This will display, for example:
Boo at line 39 (SomeMethodSomewhere)
There's also [CallerFilePath] which tells you the path of the original code file.

Use the StackFrame.GetFileLineNumber method, for example:
private static void ReportError(string message)
{
StackFrame callStack = new StackFrame(1, true);
MessageBox.Show("Error: " + message + ", File: " + callStack.GetFileName()
+ ", Line: " + callStack.GetFileLineNumber());
}
See Scott Hanselman's Blog entry for more information.
[Edit: Added the following]
For those using .Net 4.5 or later, consider the CallerFilePath, CallerMethodName and CallerLineNumber attributes in the System.Runtime.CompilerServices namespace. For example:
public void TraceMessage(string message,
[CallerMemberName] string callingMethod = "",
[CallerFilePath] string callingFilePath = "",
[CallerLineNumber] int callingFileLineNumber = 0)
{
// Write out message
}
The arguments must be string for CallerMemberName and CallerFilePath and an int for CallerLineNumber and must have a default value. Specifying these attributes on method parameters instructs the compiler to insert the appropriate value in the calling code at compile time, meaning it works through obfuscation. See Caller Information for more information.

I prefer one liners so:
int lineNumber = (new System.Diagnostics.StackFrame(0, true)).GetFileLineNumber();

In .NET 4.5 you can get the line number by creating the function:
static int LineNumber([System.Runtime.CompilerServices.CallerLineNumber] int lineNumber = 0)
{
return lineNumber;
}
Then each time you call LineNumber() you will have the current line. This has the advantage over any solution using the StackTrace that it should work in both debug and release.
So taking the original request of what is required, it would become:
MessageBox.Show("Error enter code here line number " + LineNumber());
This is building on the excellent answer by Marc Gravell.

For those who need a .NET 4.0+ method solution:
using System;
using System.IO;
using System.Diagnostics;
public static void Log(string message) {
StackFrame stackFrame = new System.Diagnostics.StackTrace(1).GetFrame(1);
string fileName = stackFrame.GetFileName();
string methodName = stackFrame.GetMethod().ToString();
int lineNumber = stackFrame.GetFileLineNumber();
Console.WriteLine("{0}({1}:{2})\n{3}", methodName, Path.GetFileName(fileName), lineNumber, message);
}
How to call:
void Test() {
Log("Look here!");
}
Output:
Void Test()(FILENAME.cs:104)
Look here!
Change the Console.WriteLine format how you like!

If its in a try catch block use this.
try
{
//Do something
}
catch (Exception ex)
{
System.Diagnostics.StackTrace trace = new System.Diagnostics.StackTrace(ex, true);
Console.WriteLine("Line: " + trace.GetFrame(0).GetFileLineNumber());
}

You only asked about the line number and with nullable project type, you then need to use a something like this
internal class Utils
{
public static int Line([CallerLineNumber] int? lineNumber =null)=>lineNumber;
}
in your code, if you like to get a line number you then just call
var line=Utils.Line();
if you are logging and you would like to document the line number in say logging than call the method like this
public void MyMethod(int someValue)
{
switch(someValue)
{
case 1:
if(abc<xyz)
{
logger.LogInformation("case value {someValue} this line {line} was true", someValue ,Utils.Line()-2);
}
break;
case 2:
logger.LogInformation("case value {someValue} this {line} was executed",someValue,Utils.Line());
break;
caste 3:
logger.LogInformation("case value {someValue} this {line} was executed",someValue,Utils.Line());
break;
}
}
You can extend this pattern with any of the other [CallerXXX] methods and not use them where ever, not just in the method parameters.
in the Nuget Package Walter I use a super cool class named ExceptionObject
if you import the NuGet package you have some nice extension methods on the Exception class as well as access to a CallStack showing the call chain including method parameters and parameter values of all methods called.
It's like a stack of an exception only with values showing how you got where you got with what values.
public void MyMethod()
{
try
{
//get me all methods, signatures, parameters line numbers file names etc used as well as assembly info of all assemblies used for documentation of how the code got here
var stack= new CallStack();
foreach( var frame in StackedFrames)
{
logger.LogDebug(frame.ToString());
}
}
catch(SqlException e)
{
var ex = new ExceptionObject(e);
logger.LogException(e,"{this} exception due to {message} {server} {procedure} TSQL-line:{sqlline}\n{TSQL}"
,e.GetType().Name
,e.Message
,ex.SqlServer
,ex.SqlProcedureName
,ex.SqlLineNumber
,ex.Tsql
,ex.CallStack);
}
catch(Exception e)
{
var ex = new ExceptionObject(e);
logger.LogException(e,"{this} exception due to {message} signature: signature}\nCallStack:", e.GetType().Name,e.Message,ex.Signature,ex.CallStack);
}
}

Is it possible to use an extension method that also has an out parameter?

Example:
public static string BoolToYesOrNo(this bool text, out string outAsHtmlName)
{
string[] choices = { "Yes", "No", "N/A" };
switch (text)
{
case true: outAsHtmlName = choices[0]; return choices[0];
case false: outAsHtmlName = choices[1]; return choices[1];
default: outAsHtmlName = choices[2]; return choices[2];
}
}
throws an exception that no overload ... takes 1 argument, altough i am using 2 arguments.
myBool.BoolToYesOrNo(out htmlClassName);
this is the exact exception: CS1501: No overload for method 'BoolToYesOrNo' takes 1 arguments.

This works fine for me with your code:
static void Main()
{
bool x = true;
string html;
string s = x.BoolToYesOrNo(out html);
}
Most likely, you are missing using directive to the namespace of the type that declares BoolToYesOrNo, so add:
using The.Correct.Namespace;
to the top of your code file, where:
namespace The.Correct.Namespace {
public static class SomeType {
public static string BoolToYesOrNo(this ...) {...}
}
}

I tried your code this way and it works without any exceptions, the only thing I would point out if you are giving a parameter with out then you don't need the method to do any return of string
bool b = true;
string htmlName;
string boolToYesOrNo = b.BoolToYesOrNo(out htmlName);

This is what I did to test this out:
I created a new C# Console Application (framework 4.5) in Visual Studio 2012 RC
Changed program.cs to look like this
(omitting usings)
namespace ConsoleApplication1
{
public static class testClass
{
public static string BoolToYesOrNo(this bool text, out string outAsHtmlName)
{
string[] choices = { "Yes", "No", "N/A" };
switch (text)
{
case true: outAsHtmlName = choices[0]; return choices[0];
case false: outAsHtmlName = choices[1]; return choices[1];
default: outAsHtmlName = choices[2]; return choices[2];
}
}
}
class Program
{
static void Main(string[] args)
{
bool b = true;
string result = string.Empty;
string retval = b.BoolToYesOrNo(out result);
Console.WriteLine(retval + ", " + result); //output: "Yes, Yes";
}
}
}
I hit F5 to run the program. The code ran perfectly. So, your method is in fact correct, and there is something wrong ... well, somewhere else. Double check the braces, sometimes if you miss one you get odd errors.

I simply paste your code and it works fine. I tried both .net 3.5 and 4.0 and no compile error is shown and the results are correct.
Why this is an overload method?

Found the answer on a MS forum, was a vs 2012 bug, and after installing the july 2012 update, everything worked fine. Thank you.