What does the 0 in "{0:MM dd yyyy}" do?

Question

I'm new to the C#/MVC world. I spend a lot of time today figuring out how to display a DateTimeOffset object in the format i want. Finally got it working this way.

Html.TextBoxFor(model => model.DeliveryDate,"{0:MM/dd/yyyy}", 
                     new { htmlAttributes = new { @class = "datepicker" } })  

But I still don't understand the importance of '0' in the format string. the page breaks if i replace the 0 with any other number or totally remove it. Can someone help me understand this?

Answer 1

From String.Format Method

The {0} in the format string is a format item. 0 is the index of the object whose string value will be inserted at that position. (Indexes start at 0.) If the object to be inserted is not a string, its ToString method is called to convert it to one before inserting it in the result string.

Answer 2

When you use the string.Format you can pass the space for arguments like {0}, {1}, etc which is the indexes you pass as arguments for the method. It is the same for asp.net razor helpers.

You also can provide the format after the index separating by :, for sample: {0:0.00} as format for a number with 2 decimals places or {1:dd/MM/yyyy} for dates etc.

String Interpolation

There is a new way to implement it using the String Interpolation. Basically, you can concat the values on your string without generating new strings. For sample:

var i = 18;
var s = $"You are {age} years old.";

Since you start the string with $, you can pass arguments between { and }. You also can use the same formats to format your data as you use on string.Format. For sample:

var today = $"Today is {DateTime.Now:D}";

var date = DateTime.Now.Add(1);
var tommorrow = $"Tommorrow is {date:dd/MM/yyyy}";

Answer 3

That's a format string with parameters (like used in e.g. Console.WriteLine, or string.Format). The {0} would be the placeholder for the first argument, and {0:mm/dd/yyyy} is simply a format string to convert the first argument to a string.

Answer 4

See the documentation for String.Format():

https://msdn.microsoft.com/en-us/library/system.string.format.aspx

In a nutshell, when the model is rendered to HTML text, the DeliveryDate object value will be passed to String.Format(), where {0} indicates the index of the first value in an array of values being passed to Format(). So {0:MM/dd/yyyy} just means to format the first value in the array using date components. Basically, it will do something like this internally:

String s = SomeValueArray[0].ToString("MM/dd/yyyy");