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Expressions in the Debugger

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Note

This article applies to Visual Studio 2015. If you're looking for the latest Visual Studio documentation, see Visual Studio documentation. We recommend upgrading to the latest version of Visual Studio. Download it here

The Visual Studio debugger includes expression evaluators that work when you enter an expression in the QuickWatch dialog box, Watch window, or Immediate window. The expression evaluators are also at work in the Breakpoints window and many other places in the debugger.

The following sections give details about expressions in different languages.

F# expressions are not supported

F# expressions are not recognized. If you are debugging F# code, you need to translate your expressions into C# syntax before entering the expressions into a debugger window or dialog box. When you translate expressions from F# to C#, be sure to remember that C# uses the == operator to test for equality, while F# uses the single =.

C++ Expressions

For information about using context operators with expressions in C++, see Context Operator (C++).

Unsupported Expressions in C++

Constructors, destructors, and conversions

You cannot call a constructor or destructor for an object, either explicitly or implicitly. For example, the following expression explicitly calls a constructor and results in an error message:

my_date( 2, 3, 1985 )

You cannot call a conversion function if the destination of the conversion is a class. Such a conversion involves the construction of an object. For example, if myFraction is an instance of CFraction, which defines the conversion function operator FixedPoint, the following expression results in an error:

(FixedPoint)myFraction

You cannot call the new or delete operators. For example, the following expression is not supported:

new Date(2,3,1985)

Preprocessor Macros

Preprocessor macros are not supported in the debugger. For instance, if a constant VALUE is declared as: #define VALUE 3, you cannot use VALUE in the Watch window. To avoid this limitation, you should replace #define's with enums and functions whenever possible.

using namespace declarations

You cannot use using namespace declarations. In order to access a type name or variable outside of the current namespace, you must use the fully-qualified name.

Anonymous namespaces

Anonymous namespaces are not supported. If you have the following code, you cannot add test to the watch window:

namespace mars   
{   
    namespace  
    {  
        int test = 0;   
    }   
}   
int main()   
{   
    // Adding a watch on test does not work.   
    mars::test++;   
    return 0;   
}

Using debugger intrinsic functions to maintain state

The debugger intrinsic functions give you a way to call certain C/C++ functions in expressions without changing the state of the application.

Debugger intrinsic functions:

  • Are guaranteed to be safe: executing a debugger intrinsic function will not corrupt the process that is being debugged.

  • Are allowed in all expressions , even in scenarios where side effects and function evaluation are not allowed.

  • Work in scenarios where the regular function calls are not possible, such as debugging a minidump.

    Debugger intrinsic functions can also make evaluating expressions more convenient. For example, strncmp(str, “asd”) is much easier to write in a breakpoint condition than str[0] == ‘a’ && str[1] == ‘s’ && str[2] == ‘d’. )

Area Intrinsic functions
String length strlen, wcslen, strnlen, wcsnlen
String comparison strcmp, wcscmp, stricmp, _stricmp, _strcmpi, wcsicmp, _wcscmpi, _wcsnicmp, strncmp, wcsncmp, strnicmp, wcsnicmp
String search strchr, wcschr, strstr, wcsstr
Win32 GetLastError(), TlsGetValue()
Windows 8 WindowsGetStringLen(), WindowsGetStringRawBuffer()

These functions require the process that is being debugged to be running on Windows 8. Debugging dump files generated from a Windows 8 device also requires that the Visual Studio computer be running Windows 8. However, if you are debugging a Windows 8 device remotely, the Visual Studio computer can be running Windows 7.
Miscellaneous __log2

Returns the log base 2 of a specified integer, rounded to the nearest lower integer.

C++/CLI - Unsupported Expressions

  • Casts that involve pointers, or user-defined casts, are not supported.

  • Object comparison and assignment are not supported.

  • Overloaded operators and overloaded functions are not supported.

  • Boxing and unboxing are not supported.

  • Sizeof operator is not supported.

C# - Unsupported Expressions

Dynamic Objects

You can use variables in debugger expressions that are statically typed as dynamic. When objects that implement the IDynamicMetaObjectProvider are evaluated in the Watch window, a Dynamic View node is added. The Dynamic View node shows object members but does not allow editing the values of the members.

The following features of dynamic objects are not supported:

  • The compound operators +=, -=, %=, /=, and *=

  • Many casts, including numeric casts and type-argument casts

  • Method calls with more than two arguments

  • Property getters with more than two arguments

  • Property setters with arguments

  • Assigning to an indexer

  • Boolean operators && and ||

Anonymous Methods

Creation of new anonymous methods is not supported.

Visual Basic - Unsupported Expressions

Dynamic Objects

You can use variables in debugger expressions that are statically typed as dynamic. When objects that implement the IDynamicMetaObjectProvider are evaluated in the Watch window, a Dynamic View node is added. The Dynamic View node shows object members but does not allow editing the values of the members.

The following features of dynamic objects are not supported:

  • The compound operators +=, -=, %=, /=, and *=

  • Many casts, including numeric casts and type-argument casts

  • Method calls with more than two arguments

  • Property getters with more than two arguments

  • Property setters with arguments

  • Assigning to an indexer

  • Boolean operators && and ||

Local Constants

Local constants are not supported.

Import Aliases

Import aliases are not supported.

Variable Declarations

You cannot declare explicit new variables in debugger windows. However, you can assign new implicit variables inside the Immediate window. These implicit variables are scoped to the debug session and are not accessible outside of the debugger. For example, the statement o = 5 implicitly creates a new variable o and assign the value 5 to it. Such implicit variables are of type Object unless the type can be inferred by the debugger.

Unsupported Keywords

  • AddressOf

  • End

  • Error

  • Exit

  • Goto

  • On Error

  • Resume

  • Return

  • Select/Case

  • Stop

  • SyncLock

  • Throw

  • Try/Catch/Finally

  • With

  • Namespace or module level keywords, such as End Sub or Module.

See Also

Format Specifiers in C++
Context Operator (C++)
Format Specifiers in C#
Pseudovariables