I am wondering if it is possible to merge ToArray conversion with resulting array single element change into one statement.

This works perfectly

var response = Enumerable.Range(1, 5).Select(index => new WeatherForecast
{
    Date = DateOnly.FromDateTime(DateTime.Now.AddDays(index)),
    TemperatureC = Random.Shared.Next(-20, 55),
    Summary = Summaries[Random.Shared.Next(Summaries.Length)]
})
.ToArray();

response[0].Summary = "Some text";

return response;

However this one, returns only string contained in Summary

return Enumerable.Range(1, 5).Select(index => new WeatherForecast
{
    Date = DateOnly.FromDateTime(DateTime.Now.AddDays(index)),
    TemperatureC = Random.Shared.Next(-20, 55),
    Summary = Summaries[Random.Shared.Next(Summaries.Length)]
})
.ToArray()[0].Summary = "Some text";

I am wondering if it is possible to merge ToArray conversion with resulting array single element change into one statement.

This works perfectly

var response = Enumerable.Range(1, 5).Select(index => new WeatherForecast
{
    Date = DateOnly.FromDateTime(DateTime.Now.AddDays(index)),
    TemperatureC = Random.Shared.Next(-20, 55),
    Summary = Summaries[Random.Shared.Next(Summaries.Length)]
})
.ToArray();

response[0].Summary = "Some text";

return response;

However this one, returns only string contained in Summary

return Enumerable.Range(1, 5).Select(index => new WeatherForecast
{
    Date = DateOnly.FromDateTime(DateTime.Now.AddDays(index)),
    TemperatureC = Random.Shared.Next(-20, 55),
    Summary = Summaries[Random.Shared.Next(Summaries.Length)]
})
.ToArray()[0].Summary = "Some text";

• c#
• arrays
• linq

• 1 it's possible. but is it necessary? – Ivan Petrov Commented Feb 16 at 20:50
• 4 Your original code is clear. I would leave it as is. – mjwills Commented Feb 16 at 21:12
• 1 Original code is more performant since the comparison is redundant, especially in large dataset – khteh Commented Feb 17 at 6:56

2 Answers 2

Well, to have Linq only solution, you can try adding ternary operator:

  Summary = index == 1 
    ? "Some text" 
    : Summaries[Random.Shared.Next(Summaries.Length)]

Code:

return Enumerable
  .Range(1, 5)
  .Select(index => new WeatherForecast {
    Date = DateOnly.FromDateTime(DateTime.Now.AddDays(index)),
    TemperatureC = Random.Shared.Next(-20, 55),
    Summary = index == 1 
      ? "Some text" // 1st item is special one
      : Summaries[Random.Shared.Next(Summaries.Length)] // business as usual
  })
  .ToArray();

Another (more general) possibility is an extra Select in the very end:

return Enumerable
  .Range(1, 5)
   ...
   /* A lot of Linq here */
   ...
  .Select((value, index) => index == 0 
     ? "Some text"
     : value)
  .ToArray();

You can always make extension methods such as this if you have to be on one line:

public static class Extensions {
    public static T[] WithChangeAt<T>(this T[] arr, int index, Action<T> change)
    where T : class {
        ArgumentNullException.ThrowIfNull(arr);
        if ((uint)index >= (uint)arr.Length)
            throw new ArgumentException("Index is out of bounds for the array");
            
        change(arr[index]);
        return arr;
    }
}

then

return Enumerable.Range(1, 5).Select(index => new WeatherForecast
{
    Date = DateOnly.FromDateTime(DateTime.Now.AddDays(index)),
    TemperatureC = Random.Shared.Next(-20, 55),
    Summary = Summaries[Random.Shared.Next(Summaries.Length)]
})
.ToArray()
.WithChangeAt(0, x => x.Summary = "Some text");