"var" type inference in C# [duplicate]

This question already has answers here: Closed 12 years ago.

Possible Duplicate:

Why does var evaluate to System.Object in “foreach (var row in table.Rows)”?

I was rather suprised to discovered the following today....

SqlDataReader reader = cmd.ExecuteReader();
DataTable schemaTable = reader.GetSchemaTable();


// the following compiles correctly
foreach (DataRow field in schemaTable.Rows)
{
    Console.WriteLine(field["ColumnName"]);
}


// the following does not compile as 'var' is of type 'object'
foreach (var field in schemaTable.Rows)
{
    // Error: Cannot apply indexing with [] to an expression of type 'object'
    Console.WriteLine(field["ColumnName"]);
}

Whats going on here?

Is this a type inference failure? And if so, what causes it?

Or is it part of the defined behaviour or var? And if so, why?

I t开发者_运维技巧hought the idea of var was that you could use it anywhere in a variable declaration/initialisation without changing behaviour.

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The point here is not var, but the foreach loop. The foreach loop can optionally cast the iterator in addition to iterating itself.

So you can do the following:

List<object> test = new List<object>();
test.Add(1);
test.Add(2);
test.Add(3);
foreach( int i in test ){
  i.Dump();
}

So even if the list is of type object, it can be casted to int on the fly inside the foreach.

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DataTable.Rows returns System.Data.DataRowCollection which is a subclass of InternalDataCollectionBase.

The GetEnumerator memthod on this returns IEnumerator, rather than IEnumerator<DataRows>.

Hence the only type information available is that it returns object, so when you specify you are enumerating DataRow you are adding your own cast, which var does not.

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The confusion is that foreach(SomeType thing in SomeCollection) doesn't just iterate through the collection, it also attempts a cast to SomeType for each item as it iterates. But with var there's nothing to cast to.