Gracefully handles the expression tree with custom Expression implementations

[CXuesong]

Describe the bug
When working with IQueryable, I have an expression tree with some custom Expression-derived class inside. I want to visualize the expression tree but it fails.

To Reproduce
Expression tree DebugView. Note there is an Extension<WikiClientLibrary.Cargo.Linq.CargoQueryRootExpression> node, which is a custom Expression class.

.Call System.Linq.Queryable.Where(
    .Call System.Linq.Queryable.Select(
        .Extension<WikiClientLibrary.Cargo.Linq.CargoQueryRootExpression>,
        '(.Lambda #Lambda1<System.Func`2[WikiClientLibrary.Tests.UnitTestProject1.Tests.CargoTests+LolSkin,<>f__AnonymousType1`3[System.String,System.String,System.DateTime]]>))
    ,
    '(.Lambda #Lambda2<System.Func`2[<>f__AnonymousType1`3[System.String,System.String,System.DateTime],System.Boolean]>))

.Lambda #Lambda1<System.Func`2[WikiClientLibrary.Tests.UnitTestProject1.Tests.CargoTests+LolSkin,<>f__AnonymousType1`3[System.String,System.String,System.DateTime]]>(WikiClientLibrary.Tests.UnitTestProject1.Tests.CargoTests+LolSkin $s)
{
    .New <>f__AnonymousType1`3[System.String,System.String,System.DateTime](
        $s.Name,
        $s.Champion,
        $s.ReleaseDate)
}

.Lambda #Lambda2<System.Func`2[<>f__AnonymousType1`3[System.String,System.String,System.DateTime],System.Boolean]>(<>f__AnonymousType1`3[System.String,System.String,System.DateTime] $s)
{
    $s.Champion == (.Constant<WikiClientLibrary.Tests.UnitTestProject1.Tests.CargoTests+<>c__DisplayClass3_0>(WikiClientLibrary.Tests.UnitTestProject1.Tests.CargoTests+<>c__DisplayClass3_0).closureParams).Champion
}

Expected behavior
I assume I should still be able to see the rest part of the expression. Perhaps you can replace the unknown expressions with some placeholders, but for now I'm completely blocked from visualizing any part of the expression.

Screenshots

Version info:

• Visual Studio version: 2019
• Visualizer location: [VS install path, or Documents subfolder]
• Version: 1.7.109

Note that the visualizer location and version can be found at the bottom of the Settings popup (click the gear icon).

Additional context
Add any other context about the problem here.

[zspitz]

@CXuesong Thanks for taking the time to report this.

I've transferred this issue here because it's a bug in the ExpressionTreeToString library, not the visualizer per se.

ValueExtractor generates values for each node in the expression tree, by wrapping it in a LambdaExpression (if it isn't a LambdaExpression) and invoking the LambdaExpression.

I'm assuming that CanReduce on the extension node returns false? AFAICT the error comes about because it cannot be reduced, and therefore any nodes which refer to it should not be evaluated either.

I've committed a fix. Could you add the latest version of ExpressionTreeToString (3.2.53 when it becomes available) to your project, and call query.Expression.ToString("Textual tree") in order to test the fix?

Also note that the fix will not immediately be available in the newest release of the visualizer. I'll let you know when that's finished.

[CXuesong]

I'm assuming that CanReduce on the extension node returns false?

Yeah you can see the definition of CargoQueryRootExpression here: https://github.com/CXuesong/WikiClientLibrary/blob/db722959e3a921a8cba448dee1a80d433aaacd7c/WikiClientLibrary.Cargo/Linq/CargoQueryRootExpression.cs

It's by design not reduciable.

[CXuesong]

I've tried your latest version and it works. Thanks for your fast fix!

https://dotnetfiddle.net/SunZVv

[zspitz]

@CXuesong The latest release of the visualizer includes the fix.