Support for the Autos window in a legacy language service

Applies to: Visual Studio Visual Studio for Mac

Note

This article applies to Visual Studio 2017. 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 Autos window displays expressions such as variables and parameters that are in scope when the program being debugged is paused (either due to a breakpoint or an exception). The expressions can include variables, local or global, and parameters that have been changed in the local scope. The Autos window can also include instantiations of a class, structure, or some other type. Anything that an expression evaluator can evaluate can potentially be shown in the Autos window.

The managed package framework (MPF) does not provide direct support for the Autos window. However, if you override the GetProximityExpressions method, you can return a list of expressions to be presented in the Autos window.

Implementing support for the Autos window

All you need to do to support the Autos window is to implement the GetProximityExpressions method in the LanguageService class. Your implementation must decide, given a location in the source file, which expressions should appear in the Autos window. The method returns a list of strings in which each string represents a single expression. A return value of S_OK indicates that the list contains expressions, while S_FALSE indicates that there are no expressions to show.

The actual expressions returned are the names of the variables or parameters that appear at that location in the code. These names are passed to the expression evaluator to obtain values and types that are then displayed in the Autos window.

Example

The following example shows an implementation of the GetProximityExpressions method that gets a list of expressions from the ParseSource method using the parse reason ParseReason. Each of the expressions is wrapped as a TestVsEnumBSTR that implements the IVsEnumBSTR interface.

Note that the GetAutoExpressionsCount and GetAutoExpression methods are custom methods on the TestAuthoringSink object and were added to support this example. They represent one way in which expressions added to the TestAuthoringSink object by the parser (by calling the AutoExpression method) can be accessed outside the parser.

using Microsoft.VisualStudio;
using Microsoft.VisualStudio.Package;
using Microsoft.VisualStudio.TextManager.Interop;

namespace TestLanguagePackage
{
    public class TestLanguageService : LanguageService
    {
        public override int GetProximityExpressions(IVsTextBuffer buffer,
                                                    int line,
                                                    int col,
                                                    int cLines,
                                                    out IVsEnumBSTR ppEnum)
        {
            int retval = VSConstants.E_NOTIMPL;
            ppEnum = null;
            if (buffer != null)
            {
                IVsTextLines textLines = buffer as IVsTextLines;
                if (textLines != null)
                {
                    Source src = this.GetSource(textLines);
                    if (src != null)
                    {
                        TokenInfo tokenInfo = new TokenInfo();
                        string text = src.GetText();
                        ParseRequest req = CreateParseRequest(src,
                                                              line,
                                                              col,
                                                              tokenInfo,
                                                              text,
                                                              src.GetFilePath(),
                                                              ParseReason.Autos,
                                                              null);
                        req.Scope = this.ParseSource(req);
                        TestAuthoringSink sink = req.Sink as TestAuthoringSink;

                        retval = VSConstants.S_FALSE;
                        int spanCount = sink.GetAutoExpressionsCount();
                        if (spanCount > 0) {
                            TestVsEnumBSTR bstrList = new TestVsEnumBSTR();
                            for (int i = 0; i < spanCount; i++)
                            {
                                TextSpan span;
                                sink.GetAutoExpression(i, out span);
                                string expression = src.GetText(span.iStartLine,
                                                                span.iStartIndex,
                                                                span.iEndLine,
                                                                span.iEndIndex);
                                bstrList.AddString(expression);
                            }
                            ppEnum = bstrList;
                            retval = VSConstants.S_OK;
                        }
                    }
                }
            }
            return retval;
        }
    }
}