Easy C# question about the Debug.Writeline() with parameters

Question

While debugging my application, I came across a behavior I'm puzzled by. I have the following

foreach (var customer in _dbContext.Customer)
{
  Debug.WriteLine("Customer Name: {0}", customer.Name);  // The output was not what I expected.
...
}

Actual Output

Peter:Customer Name: {0}

However, if I rewrite the statement to this.

foreach (var customer in _dbContext.Customer)
{
  Debug.WriteLine("Customer Name: " + customer.Name);  
...
}

Actual Output

Customer Name: Peter

I added the following the code to the same file to see why my original code wasn't working.

string first = "Peter";
string last = "Piper";
string what = "pick";
Debug.WriteLine("1 {0} 2 {1}, 3 {0} 4 {1}, 5 {2}.", first, last, what);

Actual Output

1 Peter 2 Piper, 3 Peter 4 Piper, 5 pick.

I am not sure why Debug.WriteLine("Customer Name: {0}", customer.Name); would output this CusPeter:Customer Name: {0}

Many Thanks

Different overload: learn.microsoft.com/en-us/dotnet/api/… - try Debug.WriteLine("Customer Name: {0}", (object)customer.Name) instead — Caramiriel
– Caramiriel, Commented Feb 27, 2019 at 20:10
Overload resolution rules are obscure and I wouldn't recommend memorizing them but when your types perfectly match a given signature you shouldn't be surprised that it's the one chosen. — user1228
– user1228, Commented Feb 27, 2019 at 20:13
Is there any aversion to using just straight interpolation? $"Customer Name: {customer.Name}";? — Jlalonde
– Jlalonde, Commented Feb 27, 2019 at 20:20
Thank you all. All those suggestions worked. I'm not sure why casting (object) worked though. '.Name' is already. Originally, I even tried Name.ToString and it still didn't output correctly. — PKonstant
– PKonstant, Commented Feb 27, 2019 at 20:31
Possible duplicate of Why does Debug.WriteLine incorrectly format strings? — Caramiriel
– Caramiriel, Commented Feb 27, 2019 at 20:44

Stormhashe · Accepted Answer · 2019-02-27 20:42:07Z

3

The problem happens because of the overload of the method. When you pass a string as a second parameter, it maps to the following method:

As you can see, this overload is not expecting "arguments", as you would when you want to replace them in a string, but rather is waiting for a "category" of the de Debug information.

The code casting your string to object works because of the following overload:

As you can see, when you cast your string to "object", it maps to another overload of the WriteLine method, which is actually expecting values to be formatted into a string. I believe this answers your doubt about why it works when casting to object.

answered Feb 27, 2019 at 20:42

Stormhashe

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Soviut · Accepted Answer · 2024-05-17 02:07:35Z

1

A working alternative is using $ string interpolation

Yielding this

Debug.Writline($"CustomerName {customer.Name});

I accidently answered this in comments but found out the the actual reason is answered here and that we're hitting the wrong overload, and not the one for interpolation.

edited May 17, 2024 at 2:07

Soviut

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answered Feb 27, 2019 at 20:42

Jlalonde

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