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How to check if a function throws an exception in c#?

How to check if a function throws an exception in c#?
public List<string> GetFileNames()
{
try
{
// do something
// return something
}
catch(Exception ex)
{
// do something
// log something
}
}
then i will call GetFileNames() somewhere in my code, but I want to check if it throws an exception,
like,
var list = GetFileNames(); // can be 0 count
if(GetFileNames() throws an error)
{
DoThisMethod()
}
else
{
DoThisOtherMethod();
}

You have a lot of options here:
This is generally done with a Try... pattern like TryParse.
bool TryGetFileNames(out List<string> fileNames)
You can also return null.

You can"t do this in c#.
The closest thing to what you are describing is the "checked exceptions" which are implemented in java. In such case the function will declare it is throwing some exception like so :
public void foo() throws IOException {
// your code
}
At compile time you will be forsed to take care of this by either enclosing this in TryCatch block or propagate this the same way in your function.
In c# enclose the function in TryCatch block and use different function in case of faliure.

The fundamental problem is that you're attempting to handle an exception when you're not able to do so.
If GetFilenames cannot recover from the exception, it should throw an exception itself. That may be by omitting a try/catch entirely, or by catching it, wrapping and re-throwing.
public List<string> GetFilenames() {
try {
...
} catch (Exception e) {
throw new FileLoadException("Failed to get filenames", e);
// Or if you don't want to create custom exceptions, perhaps use an InvalidOperationException
}
}
Failing that, if you don't actually need to abstract the functionality, don't catch the exception in GetFilenames at all, then call it like this:
try {
var list = GetFilenames()
DoSomething();
} catch (Exception e) {
DoSomethingElse();
}

I think you can make it simpler:
public void ICallGetFileNames()
{
var list = new List<YourObject>();
try
{
list = GetFileNames();
DoThisOtherMethod();
}
catch (Exception ex)
{
DoThisMethod();
}
}
This way, if the exception is thrown by your GetFileNames method, the DoThisOtherMethod() won't be called, since your code is going directly to the Exception block. Otherwise, if no exception is thrown, your code will call the DoThisOtherMethod just after the GetFileNames method.

Exception treatment best practice

I would like to know which is the best way to make a Exception treatment, because inside my Try statement, I have a lot of validations, and if I get some Exception there, my Catch statement can tell me what happens, but how could I know in which field occurs the Exception ?
Sample Code
try
{
// If I get a Exception when converting to number,
// I will understand the error
// but how could I know where in my `Try` statement was the error ?
int valor = Convert.ToInt32(xmlnode[i].ChildNodes.Item(2).InnerText.Trim());
// A Lot of another validations here
}
Catch(Exception e)
{
this.LogInformation(e.Message);
}

Best practises would be not to use Try-Catch at all when you convert strings to numbers. Therefore you should use the TryParse methods like int.TryParse.
// note that here is also a possible error-source
string valorToken = xmlnode[i].ChildNodes.Item(2).InnerText.Trim();
int valor;
if(!int.TryParse(valorToken, out valor))
{
// log this
}
// else valor was parsed correctly
Apart from that, if you want to provide exact error messages you have to use multiple try-catch or handle different exception types (the most general Exception type must be the last).

Don't use Convert.ToInt32 if you're unsure of the value. Use Int32.TryParse instead:
int valor;
if (Int32.TryParse(xmlnode[i].ChildNodes.Item(2).InnerText.Trim(), out valor))
{
// Worked! valor contains value
}
else
{
// Not a valid Int32
}
In addition you should not be using Exceptions to catch validation errors. Your validation code should calculate if the value is correct, rather than failing when it isn't. A validation class should expect to receive both valid and invalid data as input. Because you expect invalid input you should not be catching exceptions when it's invalid.
Come up with a test that checks if the data is valid and return true or false. Pretty much all numeric types have a TryParse method like the above. For your custom rules for other validation methods come up with a specification that defines exactly what valid and invalid input is and then write a method to implement that specification.

Move try..catch inside loop. Thus you will know which item exactly caused exception
foreach(var xmlNode in nodes)
{
try
{
//
int valor = Convert.ToInt32(xmlNode.ChildNodes.Item(2).InnerText.Trim());
// A Lot of another validations here
}
catch(Exception e)
{
LogInformation(e.Message); // current item is xmlNode
return;
}
}

If there is even the remotest possibility that the value you're tring to parse will not be parsable, it is therefore not an exceptional circumstance, vis. should not be treated as an exception.
In this case, there is TryParse, which allows you to determine that the value is not valid for parsing:
int valor;
if(int.TryParse(xmlnode[i].ChildNodes.Item(2).InnerText.Trim(), out valor))
{
// "valor" is sucessfully parsed
}
else
{
// invalid parse - do something with that knowledge
}

Unless its different Exceptions that get created (i.e. different classes) then you will need to handle this with different try catches.
Typically you can do:
try
{
// If I get a Exception when converting to number,
// I will understand the error
// but how could I know where in my `Try` statement was the error ?
int valor = Convert.ToInt32(xmlnode[i].ChildNodes.Item(2).InnerText.Trim());
// A Lot of another validations here
}
Catch(IOException ioe) {
// Handle, log
}
Catch(ArgumentNullException ane) {
// Handle, log
}
Catch(Exception e)
{
// Handle, log and potentially rethrow
}
You could also have individual try catches (which is kind of what most people would do I think) or nested try catches in your try block:
Like
// First block
try {
// Convert here once
} catch (Exception ex) {
// Handle and log
}
// Second block
try {
// Convert here once
} catch (Exception ex) {
// Handle and log
}
Not sure if that helps at all.

try
{
}
catch (Exception ex)
{
var stackTrace = new StackTrace(ex, true);
var frame = stackTrace.GetFrame(0);
var line = frame.GetFileLineNumber();
var method = frame.GetMethod();
}

How to determine if an exception is of a particular type

I have a piece of try catch code:
try
{
...
}
catch(Exception ex)
{
ModelState.AddModelError(
"duplicateInvoiceNumberOrganisation", "The combination of organisation and invoice number must be unique");
}
For this piece of code I'm trying to insert a record into a database: The dba has set it up so that the database checks for duplicates and returns an error if there are duplicates. Currently, as you can see, I'm adding the same error to the model no matter what error occurred. I want it changed so this error is only added to the model if it was caused by the duplicate error set up by the dba.
Below is the error I want to catch. Note it's in the inner exception. Can anyone tell me how to specifically catch this one?

before your current catch add the following:
catch(DbUpdateException ex)
{
if(ex.InnerException is UpdateException)
{
// do what you want with ex.InnerException...
}
}
From C# 6, you can do the following:
catch(DbUpdateException ex) when (ex.InnerException is UpdateException)
{
// do what you want with ex.InnerException...
}

Replace System.Threading.ThreadAbortException with your exception.
try
{
//assume ThreadAbortException occurs here
}
catch (Exception ex)
{
if (ex.GetType().IsAssignableFrom(typeof(System.Threading.ThreadAbortException)))
{
//what you want to do when ThreadAbortException occurs
}
else
{
//do when other exceptions occur
}
}

Not enough rep to comment. In response to #conterio question (in #Davide Piras answer):
is there a catch "when not" syntax?
There is.
catch (Exception e) when (!(e is ArgumentException)) { }

To get name of the exception you can use
catch (Exception exc){
if (exc.GetType().FullName == "Your_Exception")
{
// The same can be user for InnerExceptions
// exc.InnerException.GetType().FullName
}
}

You can take a look at the SQLException class -- and check for the contents of the exception's message if it contains what you now see in your inner exception..Something like this:
try
{
//your code here
}
catch (SQLException ex)
{
if (ex.Message.Contains("Cannot insert duplicate key in obj...."))
{
//your code here
}
}

How Can I Force Execution to the Catch Block?

I am wondering can try..catch force execution to go into the catch and run code in there?
here example code:
try {
if (AnyConditionTrue) {
// run some code
}
else {
// go catch
}
} catch (Exception) {
// run some code here...
}

try{
if (AnyConditionTrue){
//run some code
}
else{
throw new Exception();
}
}
catch(){
//run some code here...
}
But like Yuck has stated, I wouldn't recommend this. You should take a step back at your design and what you're looking to accomplish. There's a better way to do it (i.e. with normal conditional flow, instead of exception handling).

Rather than throwing an Exception in the else, I would recommend extracting the code from your catch into a method and call that from your else
try
{
if (AnyConditionTrue)
{
MethodWhenTrue();
}
else
{
HandleError();
}
}
catch(Exception ex)
{
HandleError();
}

Yes, you have to throw exception :
try
{
throw new Exception("hello");
}
catch (Exception)
{
//run some code here...
}

An effective way to throw an Exception and also jump to Catch as so:
try
{
throw new Exception("Exception Message");
}
catch (Exception e)
{
// after the throw, you will land here
}

if(conditiontrue)
{
}
else{
throw new Exception();
}

Yes, if you throw the exception that you intend to catch from within the try, it will be caught in the catch section.
I have to ask you why you would want to do this though? Exception handling is not meant to be a substitute for control flow.

I think what you want is a finally block: http://msdn.microsoft.com/en-us/library/zwc8s4fz(v=vs.80).aspx
see this
try
{
doSomething();
}
catch
{
catchSomething();
throw an error
}
finally
{
alwaysDoThis();
}
This is different if/when you do this:
try
{
doSomething();
}
catch
{
catchSomething();
throw an error
}
alwaysDoThis();// will not run on error (in the catch) condition
the the this last instance, if an error occurs, the catch will execute but NOT the alwaysDoThis();. Of course you can still have multiple catch as always.

As cadrel said, but pass through an Exception to provide more feedback, which will be shown in the innerException:
try
{
if (AnyConditionTrue)
{
MethodWhenTrue();
}
else
{
HandleError(new Exception("AnyCondition is not true"));
}
}
catch (Exception ex)
{
HandleError(ex);
}
...
private void HandleError(Exception ex) {
throw new ApplicationException("Failure!", ex);
}

public class CustomException: Exception
{
public CustomException(string message)
: base(message) { }
}
//
if(something == anything)
{
throw new CustomException(" custom text message");
}
you can try this

You could throw an exception to force a catch
throw new Exception(...);

why are you catching an exception? Why not just run the code in your "else" block? If you MUST do it that way, just throw a new exception
throw new Exception();

Slight resurrection, but I wanted to add both a sample (primarily like others) and a use case.
public int GetValueNum(string name)
{
int _ret = 0;
try
{
Control c = (extendedControls.Single(s => s.ValueName == name) as Control);
if (c.GetType() == typeof(ExtendedNumericUpDown))
_ret = (int)((ExtendedNumericUpDown)c).Value;
else
throw new Exception();
}
catch
{
throw new InvalidCastException(String.Format("Invalid cast fetching .Value value for {0}.\nExtendedControllerListener.GetValueNum()", name));
}
return _ret;
}
In my case, I have custom controls - a handful of controls that use a base Windows.Forms control, but add two bools and a string for tracking, and also automatically get registered to a Singleton List<T> so they can be properly fetched without drilling down through control containers (it's a tabbed form).
In this case, I'm creating some methods to easily get values (.Value, .Text, .Checked, .Enabled) by a name string. In the case of .Value, not all Control objects have it. If the extended control is not of type ExtendedNumericUpDown, it IS an InvalidCastException as the method should not be called against that type of control. This isn't flow, but the prescribed usage of invalid cast. Since Control doesn't naturally have a .Value property, Visual Studio won't let me just force an attempt and fail after.

How to get Exception Error Code in C#

try
{
object result = processClass.InvokeMethod("Create", methodArgs);
}
catch (Exception e)
{
// Here I was hoping to get an error code.
}
When I invoke the above WMI method I am expected to get Access Denied. In my catch block I want to make sure that the exception raised was indeed for Access Denied. Is there a way I can get the error code for it ? Win32 error code for Acceess Denied is 5.
I dont want to search the error message for denied string or anything like that.
Thanks

You can use this to check the exception and the inner exception for a Win32Exception derived exception.
catch (Exception e) {
var w32ex = e as Win32Exception;
if(w32ex == null) {
w32ex = e.InnerException as Win32Exception;
}
if(w32ex != null) {
int code = w32ex.ErrorCode;
// do stuff
}
// do other stuff
}
Starting with C# 6, when can be used in a catch statement to specify a condition that must be true for the handler for a specific exception to execute.
catch (Win32Exception ex) when (ex.InnerException is Win32Exception) {
var w32ex = (Win32Exception)ex.InnerException;
var code = w32ex.ErrorCode;
}
As in the comments, you really need to see what exception is actually being thrown to understand what you can do, and in which case a specific catch is preferred over just catching Exception. Something like:
catch (BlahBlahException ex) {
// do stuff
}
Also System.Exception has a HRESULT
catch (Exception ex) {
var code = ex.HResult;
}
However, it's only available from .NET 4.5 upwards.

Building on Preet Sangha's solution, the following should safely cover the scenario where you're working with a large solution with the potential for several Inner Exceptions.
try
{
object result = processClass.InvokeMethod("Create", methodArgs);
}
catch (Exception e)
{
// Here I was hoping to get an error code.
if (ExceptionContainsErrorCode(e, 10004))
{
// Execute desired actions
}
}
...
private bool ExceptionContainsErrorCode(Exception e, int ErrorCode)
{
Win32Exception winEx = e as Win32Exception;
if (winEx != null && ErrorCode == winEx.ErrorCode)
return true;
if (e.InnerException != null)
return ExceptionContainsErrorCode(e.InnerException, ErrorCode);
return false;
}
This code has been unit tested.
I won't harp too much on the need for coming to appreciate and implement good practice when it comes to Exception Handling by managing each expected Exception Type within their own blocks.

You should look at the members of the thrown exception, particularly .Message and .InnerException.
I would also see whether or not the documentation for InvokeMethod tells you whether it throws some more specialized Exception class than Exception - such as the Win32Exception suggested by #Preet. Catching and just looking at the Exception base class may not be particularly useful.

I suggest you to use Message Properte from The Exception Object Like below code
try
{
object result = processClass.InvokeMethod("Create", methodArgs);
}
catch (Exception e)
{
//use Console.Write(e.Message); from Console Application
//and use MessageBox.Show(e.Message); from WindowsForm and WPF Application
}

catch (Exception e)
{
if (e is MyCustomExeption myEx)
{
var errorCode = myEx.ErrorCode;
}
}

Another method would be to get the error code from the exception class directly. For example:
catch (Exception ex)
{
if (ex.InnerException is ServiceResponseException)
{
ServiceResponseException srex = ex.InnerException as ServiceResponseException;
string ErrorCode = srex.ErrorCode.ToString();
}
}