Provide a reason why implicit typing should be avoided in loop variable in foreach loops

[NagayamaToshiaki]

According to C# Coding Conventions, "Don't use implicit typing to determine the type of the loop variable in foreach loops."
But no clear reason is written here, so I don't know why we should use foreach (char ch in laugh) instead of foreach (var ch in laugh).
You can find usually easily which type the var is, as Intellisense suggests like other vars.

Dictionary<Foo, Bar> X = new();
// Add some elements to X...
foreach(KeyValuePair<Foo, Bar> Y in X)
{
// Some Process here
}

Above might be difficult to guess, but that is what the C# implementation of Dictionary is, so not hard to remember.

Is this rule relevant to the note below it("Be careful not to accidentally change a type of an element of the iterable collection...")?

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• ID: 40f42506-f4a5-859a-0266-ded002056d83
• Version Independent ID: f940f2ad-e13d-4d37-6439-f8afcbc4e4c7
• Content: C# Coding Conventions
• Content Source: docs/csharp/fundamentals/coding-style/coding-conventions.md
• Product: dotnet-csharp
• Technology: csharp-fundamentals
• GitHub Login: @BillWagner
• Microsoft Alias: wiwagn

[BillWagner]

Thanks for asking this @NagayamaToshiaki

In your examples, and most instances using foreach, the type of elements in the collection isn't obvious.

This guidance follows from the general guidance on using var when the type of variable is obvious.

I'll leave this open and add the "up for grabs" label if any community member wants to update this before we make our next updates.

[paulomorgado]

I'm sorry @BillWagner, but, as stated, var should only be used with constants. And that's wrong.

How about LINQ and anonymous types?

var list = new[] {
	new { FirstName = "Paulo", LastName = "Morgado" },
	new { FirstName = "Bill", LastName = "Wagner" },
};

var orderedList = list.OrderBy(i => i.LastName).ThenBy(i => i.FirstName);

foreach (var item in orderedList)
{
	Console.WriteLine(item.ToString());
}