Detailed analysis of the use of variable parameters of C# params

Today, when calling a method that uses params mutable parameters in a .NET Core project, a null reference exception was triggered. It was originally thought that the method did not check the parameters null value, so the check null code was added:

public static void BuildBlogPostLinks(params BlogPostDto[] blogPosts)
{
 if (blogPosts == null)
  return;

 foreach (var blogPost in blogPosts)
 {
  //...
 }
}

The result was unexpected. The null reference exception continued. After carefully looking at the exception stack, I found that the null reference exception was thrown during foreach. You need to check null for blogPost during foreach.

The following sample code can verify this

class Program
{
 static void Main(string[] args)
 {
  BuildBlogPostLinks(null);
  BlogPost blogPost = null;
  BuildBlogPostLinks(blogPost);
 }

 public static void BuildBlogPostLinks(params BlogPost[] blogPosts)
 {
  if (blogPosts == null)
  {
   ("blogPosts in null");
   return;
  }

  foreach (var blogPost in blogPosts)
  {
   if (blogPost == null)
   {
    ("blogPost in null");
   }
   else 
   {
    (": " + );
   }
  }
 }
}

public class BlogPost
{
 public string Title { get; set; }
}

The output result at runtime is

$ dotnet run
blogPosts in null
blogPost in null

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