Question

I’m trying to execute the code below (in LINQPad), and my LINQ statement is getting an error

Cannot implicitly convert type System.Collections.Generic.IEnumerable<UserQuery.ResultCode> to UserQuery.ResultCode.

Why?

Here is the code:

public enum ResultCode {
    Big,
    Bad,
    Messy
}

public class ValidationResult {
    public bool IsValid = false;
    public ResultCode Code;

    public override string ToString(){
        return String.Format("IsValid: {0}, Code: {1}",IsValid, Code);
    }
}

public class RuleMap {
    public Func<bool> Fact;
    public ResultCode Code;
    public List<int> Actions;
}


public class RulesProcessor{

    List<RuleMap> rules;

    public RulesProcessor(){
        rules = new List<RuleMap>{
            new RuleMap {Fact = CheckLeft, Code = ResultCode.Big,
                Actions = new List<int> {2, 3}
            },
            new RuleMap {Fact = CheckRight, Code = ResultCode.Bad,
                Actions = new List<int> {1, 2}
            },
            new RuleMap {Fact = CheckDown, Code = ResultCode.Messy,
                Actions = new List<int> {1, 3}
            },
        };
    }

    public bool CheckLeft(){
        return true;
    }

    public bool CheckRight(){
        return false;
    }

    public bool CheckDown(){
        return true;
    }

    public ValidationResult EvaluateRulesOnAction(int actionType){

        ValidationResult result = new ValidationResult();

        ResultCode badRule = from foo in rules
                                where !foo.Fact() && foo.Actions.Contains(actionType)
                                select foo.Code;


        if (badRule != null){
            result.Code = badRule;
        }else{
            result.IsValid = true;
        }

        return result;
    }

}

void Main()
{
    var foo = new RulesProcessor();

    foo.EvaluateRulesOnAction(1).Dump();
    foo.EvaluateRulesOnAction(3).Dump();
}

Answer 1

The reason is that every LINQ query of the form from x in y select x returns an IEnumerable<TypeOfX>.

If you are sure that there can be a maximum of one bad rule, you can change your code to this:

ResultCode badRule = (from foo in rules 
                      where !foo.Fact() && foo.Actions.Contains(actionType) 
                      select foo.Code).SingleOrDefault(); 

If there can be more than one bad rule but you are only interested in the first one, use this:

ResultCode badRule = (from foo in rules 
                      where !foo.Fact() && foo.Actions.Contains(actionType) 
                      select foo.Code).FirstOrDefault(); 

The “OrDefault” in both queries indicates that you want to have the default value of the returned type. For reference types this is null.

However, in your code, the query returns an enum of type ResultCode. Enums can’t be null, so the if that comes after the query will never yield the expected results as badRule == null is always false.

You should change your code to this:

var badRule = (from foo in rules 
               where !foo.Fact() && foo.Actions.Contains(actionType) 
               select foo).FirstOrDefault(); 

if (badRule != null){
    result.Code = badRule.Code;
}else{
    result.IsValid = true;
}