C#: Missed FirstOrDefault query.

[michaelnebel]

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csharp/ql/src/Linq/MissedFirstOrDefaultOpportunity.qhelp

Missed opportunity to use FirstOrDefault

Programmers sometimes search a sequence by iterating over each element, testing it, and returning the first element that satisfies the test. If the loop completes without finding a match, the method then returns a default value such as null or default.

Recommendation

This pattern is directly available as the FirstOrDefault method in LINQ. Using the library method makes the search intent explicit and avoids manually spelling out the loop and fallback return.

Example

In this example the method searches a list of operations for the first operation with a matching identifier, returning null if no match is found.

using System;
using System.Collections.Generic;

class MissedFirstOrDefaultOpportunity
{
    public static Operation FindOperation(IEnumerable<Operation> operations, string operationId)
    {
        foreach (var operation in operations)
        {
            if (string.Equals(operation.OperationId, operationId, StringComparison.Ordinal))
                return operation;
        }

        return null;
    }
}

class Operation
{
    public string OperationId { get; set; }
}

The LINQ FirstOrDefault method can express this search more directly.

using System;
using System.Collections.Generic;
using System.Linq;

class MissedFirstOrDefaultOpportunityFix
{
    public static Operation FindOperation(IEnumerable<Operation> operations, string operationId)
    {
        return operations.FirstOrDefault(operation =>
            string.Equals(operation.OperationId, operationId, StringComparison.Ordinal));
    }
}

The following examples should not use FirstOrDefault, because they do more than return the matching element or because the fallback value is not the default value.

using System;
using System.Collections.Generic;

class MissedFirstOrDefaultOpportunityGood
{
    public static Operation FindOperationOrThrow(IEnumerable<Operation> operations, string operationId)
    {
        foreach (var operation in operations)
        {
            if (string.Equals(operation.OperationId, operationId, StringComparison.Ordinal))
                throw new InvalidOperationException("Unexpected operation.");
        }

        return null;
    }

    public static Operation FindReplacementOperation(IEnumerable<Operation> operations, string operationId)
    {
        foreach (var operation in operations)
        {
            if (string.Equals(operation.OperationId, operationId, StringComparison.Ordinal))
                return operation;
        }

        return new Operation();
    }

    public static string FindOperationId(IEnumerable<Operation> operations, string operationId)
    {
        foreach (var operation in operations)
        {
            if (string.Equals(operation.OperationId, operationId, StringComparison.Ordinal))
                return operation.OperationId;
        }

        return null;
    }
}

References

• MSDN: Enumerable.FirstOrDefault Method.