A lambda expression with a expression body cannot be converted to an expression tree C# entity frame-CodePudding

I have an entity table with below model

   public class VulnerabilityImportFileLog
    {
        public Collection<Vulnerability> Vulnerability { get; set; }
    }

Trying to return an string value using the select statement in the entity framework query

var vulnerabilityImportFileLogSelect = vulnerabilityImportFileLogQuery
        .Select(vulnerabilityImportFileLog =>
            new VulnerabilityImportedScansListViewDto
            {
                AssetName = vulnerabilityImportFileLog.Vulnerability.Select(y =>
                    {
                        if (y.AssetHostIPOrDomain.HostIPAssetId.HasValue)
                            return y.AssetHostIPOrDomain.HostIPAsset.Title;
                        else if (y.AssetHostIPOrDomain.DomainAssetId.HasValue)
                            return y.AssetHostIPOrDomain.DomainAsset.Title;
                        else
                            return y.AssetHostIPOrDomain.HostIPOrDomain;
                    }
                ).Distinct().ToList()
            });

Where AssetName is of type List<string> and getting an exception as A lamda expression with a statement body cannot be converted to an expression tree

An suggested answer was to use AsEnumerable(), but even that not solved the issue.

CodePudding user response：

You can solve the problem by using linq query syntax with 'let' keyword:

AssetName = (from y in vulnerabilityImportFileLog.Vulnerability
    let resultString = y.AssetHostIPOrDomain.HostIPAssetId.HasValue
      ? y.AssetHostIPOrDomain.HostIPAsset.Title
      : y.AssetHostIPOrDomain.DomainAssetId.HasValue
      ? y.AssetHostIPOrDomain.DomainAsset.Title
      : y.AssetHostIPOrDomain.HostIPOrDomain
  select resultString
)
.ToList()
.Distinct();

CodePudding user response：

Assuming vulnerabilityImportFileLogQuery is an IQueryable<vulnerabilityImportFileLog> then a typical way to fetch details from the associated Vulnerability relations would be via a SelectMany. Given you need to do substitution depending on what is available, one option would be to do a double-projection. The first pass selects and materializes just the details we want to inspect, then the second pass composes the results.

vulnerabilityImportFileLogQuery.SelectMany(x => new 
{
    x.Vulnerability.AssetHostIpOrDomain.HostIpOrDomain,
    HostIpAssetTitle = x.Vulnerability.AssetHostIpOrDomain.HostIpAsset.Title,
    DomainAssetTitle = x.Vulnerability.AssetHostIpOrDomain.DomainAsset.Title
}).ToList()
.Select(x => x.HostIpAssetTitle ?? DomainAssetTitle ?? HostIpOrDomain)
.Distinct()
.ToList();

What this essentially does is fetch the 3 values, our default HostIpOrDomain value, and the Titles from the HostIpAsset and DomainAsset if they are available. EF will return #null if the entities aren't associated or the Title is #null. This query is materialized using the ToList, then from that we Select the title to use based on a null check, before taking our Distinct set.

Hopefully that gives you a few ideas as a starting point.