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C# confuse parameter object and enumerator [duplicate]

First, a bit of background. Read the question and accepted answer posted here for a specific scenario for my question. I'm not sure if other, similar cases exist but this is the only case I am aware of.
The above "quirk" is something that I've been aware of for a long time. I didn't understand the full breadth of the cause until just recently.
Microsoft's documentation on the SqlParameter class sheds a little more light on the situation.
When you specify an Object in the value parameter, the SqlDbType is
inferred from the Microsoft .NET Framework type of the Object.
Use caution when you use this overload of the SqlParameter constructor
to specify integer parameter values. Because this overload takes a
value of type Object, you must convert the integral value to an Object
type when the value is zero, as the following C# example demonstrates.
Parameter = new SqlParameter("#pname", Convert.ToInt32(0));
If you do
not perform this conversion, the compiler assumes that you are trying
to call the SqlParameter (string, SqlDbType) constructor overload.
(emph. added)
My question is why does the compiler assume that when you specify a hard coded "0" (and only the value "0") that you are trying to specify an enumeration type, rather than an integer type? In this case, it assumes that you are declaring SqlDbType value, instead of the value 0.
This is non-intuitive and, to make matters worse, the error is inconsistent. I have old applications that I've written which have called stored procedures for years. I'll make a change to the application (often times not even associated with my SQL Server classes), publish an update, and this issue will all of a sudden break the application.
Why is the compiler confused by the value 0, when an object containing multiple method signatures contain two, similar signatures where one parameter is an object/integer and the other accepts an enumeration?
As I've mentioned, I've never seen this as a problem with any other constructor or method on any other class. Is this unique to the SqlParameter class or is this a bug inherit within C#/.Net?

It's because a zero-integer is implicitly convertible to an enum:
enum SqlDbType
{
Zero = 0,
One = 1
}
class TestClass
{
public TestClass(string s, object o)
{ System.Console.WriteLine("{0} => TestClass(object)", s); }
public TestClass(string s, SqlDbType e)
{ System.Console.WriteLine("{0} => TestClass(Enum SqlDbType)", s); }
}
// This is perfectly valid:
SqlDbType valid = 0;
// Whilst this is not:
SqlDbType ohNoYouDont = 1;
var a1 = new TestClass("0", 0);
// 0 => TestClass(Enum SqlDbType)
var a2 = new TestClass("1", 1);
// => 1 => TestClass(object)
(Adapted from Visual C# 2008 Breaking Changes - change 12)
When the compiler performs the overload resolution 0 is an Applicable function member for both the SqlDbType and the object constructors because:
an implicit conversion (Section 6.1) exists from the type of the argument to the type of the corresponding parameter
(Both SqlDbType x = 0 and object x = 0 are valid)
The SqlDbType parameter is better than the object parameter because of the better conversion rules:
If T1 and T2 are the same type, neither conversion is better.
object and SqlDbType are not the same type
If S is T1, C1 is the better conversion.
0 is not an object
If S is T2, C2 is the better conversion.
0 is not a SqlDbType
If an implicit conversion from T1 to T2 exists, and no implicit conversion from T2 to T1 exists, C1 is the better conversion.
No implicit conversion from object to SqlDbType exists
If an implicit conversion from T2 to T1 exists, and no implicit conversion from T1 to T2 exists, C2 is the better conversion.
An implicit conversion from SqlDbType to object exists, so the SqlDbType is the better conversion
Note that what exactly constitutes a constant 0 has (quite subtly) changed in Visual C# 2008 (Microsoft's implementation of the C# spec) as #Eric explains in his answer.

RichardTowers' answer is excellent, but I thought I'd add a bit to it.
As the other answers have pointed out, the reason for the behaviour is (1) zero is convertible to any enum, and obviously to object, and (2) any enum type is more specific that object, so the method that takes an enum is therefore chosen by overload resolution as the better method. Point two is I hope self-explanatory, but what explains point one?
First off, there is an unfortunate deviation from the specification here. The specification says that any literal zero, that is, the number 0 actually literally appearing in the source code, may be implicitly converted to any enum type. The compiler actually implements that any constant zero may be thusly converted. The reason for that is because of a bug whereby the compiler would sometimes allow constant zeroes and sometimes not, in a strange and inconsistent manner. The easiest way to solve the problem was to consistently allow constant zeroes. You can read about this in detail here:
https://web.archive.org/web/20110308161103/http://blogs.msdn.com/b/ericlippert/archive/2006/03/28/the-root-of-all-evil-part-one.aspx
Second, the reason for allowing zeros to convert to any enum is to ensure that it is always possible to zero out a "flags" enum. Good programming practice is that every "flags" enum have a value "None" which is equal to zero, but that is a guideline, not a requirement. The designers of C# 1.0 thought that it looked strange that you might have to say
for (MyFlags f = (MyFlags)0; ...
to initialize a local. My personal opinion is that this decision has caused more trouble than it was worth, both in terms of the grief over the abovementioned bug and in terms of the oddities it introduces into overload resolution that you have discovered.
Finally, the designers of the constructors could have realized that this would be a problem in the first place, and made the signatures of the overloads such that the developer could clearly decide which ctor to call without having to insert casts. Unfortunately this is a pretty obscure issue and so a lot of designers are unaware of it. Hopefully anyone reading this will not make the same mistake; do not create an ambiguity between object and any enum if you intend the two overrides to have different semantics.

This is apparently a known behavior and affects any function overloads where there is both an enumeration and object type. I don't understand it all, but Eric Lippert summed it up quite nicely on his blog

This is caused by the fact that the integer literal 0 has an implicit conversion to any enum type. The C# specification states:
6.1.3 Implicit enumeration conversions
An implicit enumeration conversion permits the decimal-integer-literal
0 to be converted to any enum-type and to any nullable-type whose
underlying type is an enum-type. In the latter case the conversion is
evaluated by converting to the underlying enum-type and wrapping the
result.
As a result, the most specific overload in this case is SqlParameter(string, DbType).
This does not apply for other int values, so the SqlParameter(string, object) constructor is the most specific.

When resolving the type for an overloaded method, C# selects the most specific option. The SqlParameter class has two constructors that take exactly two arguments, SqlParameter(String, SqlDbType) and SqlParameter(String, Object). When you provide the literal 0, it can be interpreted as an Object or as a SqlDbType. Since SqlDbType is more specific than Object, it is assumed to be the intent.
You can read more about overload resolution in this answer.

Ambiguous method overloading

This issue has caught me out once again. Could someone provide a technical explanation as to why the following code does not produce any warnings or errors. The question you have to ask yourself is (of course) do you feel lucky?
class Program
{
static string Feeling(object o) { return "Lucky"; }
static string Feeling(string s) { return "Unlucky"; }
static void Main(string[] args)
{
Console.WriteLine("I feel " + Feeling(null));
}
}
Bonus points awarded if you know which method will be called without running the code.
And just to add insult, it doesn't just happen with null parameters:
class Program
{
static string Feeling(int i) { return "Lucky"; }
static string Feeling(uint i) { return "Unlucky"; }
static void Main(string[] args)
{
Console.WriteLine("I feel " + Feeling(7));
}
}

First example: null argument
In the first case it will call the string overload. null matches both object and string, but string is the more specific/derived type. Thus it chooses string.
Check Eric Lippert's post How does the method overload resolution system decide which method to call when a null value is passed? for a longer explanation for this part of overload resolution.
Now we must determine the best of the applicable candidates. The bestness rules are complicated, but the short version is that more specific is better than less specific.
Second example: integer literal
In the second case it'll choose the first overload, because the literal 7 is int. If you had used 7u it would have been uint and thus the second overload would be preferred.
Integer literals have a well defined type(even if they allow more implicit conversions than normal integral values). You can use suffixes like u for unsigned, or l for long to influence that type. Or you can add an explicit cast.
While normally an int wouldn't be implicitly convertible to uint, this is an integer constant which is in the valid range of uint, and the C# compiler has an extra rule to allow implicit conversions between integer constants, provided the constant fits the target's range.
One again Eric explains the details: Why does this implicit conversion from int to uint work?
A constant expression of type int can be converted to type sbyte, byte, short, ushort, uint, or ulong, provided the value of the constant-expression is within the range of the destination type. A constant expression of type long can be converted to type ulong, provided the value of the constant expression is not negative.
In both examples one overload is clearly the best, as far as the C# compiler is concerned, and thus you don't get an ambiguous overloading error.
Personally I think that the first example should give a warning, but either the C# team disagrees, or they simply didn't have time to add that heuristic.

The simple answer is that it doesn't give any errors or warnings because it's entirely valid code by the C# spec.
The relevant section of the C# 4 spec is 7.5.3 in general (for the whole process), and 7.5.3.2 to determine which applicable function member is better when the first phase has found more than one. (The subsequent sections such as 7.5.3.5 give details about "better conversion targets" etc.)
Trying to explain the rules absolutely correctly but in a short space would be hard to say the least, unfortunately. I suggest you look through that bit of the spec yourself very carefully.

Strange (possibly wrong?) C# compiler behavior with method overloading and enums

today I discovered a very strange behavior with C# function overloading. The problem occurs when I have a method with 2 overloads, one accepting Object and the other accepting Enum of any type. When I pass 0 as parameter, the Enum version of the method is called. When I use any other integer value, the Object version is called. I know this can be easilly fixed by using explicit casting, but I want to know why the compiler behaves that way. Is this a bug or just some strange language rule I don't know about?
The code below explains the problem (checked with runtime 2.0.50727)
Thanks for any help on this,
Grzegorz Kyc
class Program
{
enum Bar
{
Value1,
Value2,
Value3
}
static void Main(string[] args)
{
Foo(0);
Foo(1);
Console.ReadLine();
}
static void Foo(object a)
{
Console.WriteLine("object");
}
static void Foo(Bar a)
{
Console.WriteLine("enum");
}
}

It may be that you're not aware that there's an implicit conversion from a constant1 of 0 to any enum:
Bar x = 0; // Implicit conversion
Now, the conversion from 0 to Bar is more specific than the conversion from 0 to object, which is why the Foo(Bar) overload is used.
Does that clear everything up?
1 There's actually a bug in the Microsoft C# compiler which lets it be any zero constant, not just an integer:
const decimal DecimalZero = 0.0m;
...
Bar x = DecimalZero;
It's unlikely that this will ever be fixed, as it could break existing working code. I believe Eric Lippert has a two blog posts which go into much more detail.
The C# specification section 6.1.3 (C# 4 spec) has this to say about it:
An implicit enumeration conversion
permits the decimal-integer-literal 0
to be converted to any enum-type and
to any nullable-type whose underlying
type is an enum-type. In the latter
case the conversion is evaluated by
converting to the underlying enum-type
and wrapping the result (§4.1.10).
That actually suggests that the bug isn't just in allowing the wrong type, but allowing any constant 0 value to be converted rather than only the literal value 0.
EDIT: It looks like the "constant" part was partially introduced in the C# 3 compiler. Previously it was some constant values, now it looks like it's all of them.

I know I have read somewhere else that the .NET system always treats zero as a valid enumeration value, even if it actually isn't. I will try to find some reference for this...
OK, well I found this, which quotes the following and attributes it to Eric Gunnerson:
Enums in C# do dual purpose. They are used for the usual enum use, and they're also used for bit fields. When I'm dealing with bit fields, you often want to AND a value with the bit field and check if it's true.
Our initial rules meant that you had to write:
if ((myVar & MyEnumName.ColorRed) != (MyEnumName) 0)
which we thought was difficult to read. One alernative was to define a zero entry:
if ((myVar & MyEnumName.ColorRed) != MyEnumName.NoBitsSet)
which was also ugly.
We therefore decided to relax our rules a bit, and permit an implicit conversion from the literal zero to any enum type, which allows you to write:
if ((myVar & MyEnumName.ColorRed) != 0)
which is why PlayingCard(0, 0) works.
So it appears that the whole reason behind this was to simply allow equating to zero when checking flags without having to cast the zero.

Why do Java and C# not have implicit conversions to boolean?

Since I started Java it's been very aggravating for me that it doesn't support implicit conversions from numeric types to booleans, so you can't do things like:
if (flags & 0x80) { ... }
instead you have to go through this lunacy:
if ((flags & 0x80) != 0) { ... }
It's the same with null and objects. Every other C-like language I know including JavaScript allows it, so I thought Java was just moronic, but I've just discovered that C# is the same (at least for numbers, don't know about null/objects):
http://msdn.microsoft.com/en-us/library/c8f5xwh7(VS.71).aspx
Microsoft changed it on purpose from C++, so why? Clearly I'm missing something. Why change (what I thought was) the most natural thing in the world to make it longer to type? What on Earth is wrong with it?

For clarity. It makes the following mistake simply illegal:
int x = ...;
if (x = 0) // in C: assign 0 to x and always evaluate to false
.... // never executed
Note: most modern C / C++ compilers will give a Warning (but not an Error) on this straightforward pattern, but there are many variations possible. It can creep up on you.

Both Java and C# abandoned implicit conversions to booleans to reduce the chance of programmer error.
For example, many programmers would accidentally write:
if( x = 5 ) { ... }
instead of:
if( x == 5 ) { ... }
Which of course results in completely different behavior, since the first statement performs an assignment (which will always result in true), while the second performs a comparison. In the past, developers would sometimes write such assignments in reverse to avoid the pitfall, since:
if( 5 = x ) { ... } // doesn't compile.
Now, in C#, you can still create implicit conversion operators to bool for your own types - although it is rarely advisable, since most developers don't expect it:
public class MyValue
{
public int Value { get; set; }
public static implicit operator bool( MyValue mb )
{
return mb.Value != 0;
}
}
MyValue x = new MyValue() { Value = 10; }
if( x ) { ... } // perfectly legal, compiler applies implicit conversion

Maybe they felt that being more explicit was more in line with a strongly typed language.

You've got it backward.
It's actually C that does not support boolean, so if ( and any other conditional statement ) actually expects an int value, not boolean. Then int value of 0 is treated as false and any other value is treated as true.
Some people actually find it a little ambiguous, because this type of behavior can lead to many errors, as others have pointed out. Because of this, Java designers have opted out to support only boolean types in the condition statements. And when Microsoft decided to implement MS-Java ( AKA C# ), they've borrowed this design principal.
If you don't like it, you can program in a variety of languages that do not have this restriction.

Implicit conversion of any int value (such as (flags & 0x80)) to a boolean implies a language defined mapping from an int value to a boolean. C did this, and caused a huge amount of confusion and a lot of programmer error. There is no good reason why a zero int value ALWAYS means true (or false) and a lot of good reasons why you might want to leave the decision to the programmer. For these reasons implicit conversion to boolean has been abandoned by most modern languages.
If typing seven extra characters every time you do a bit test constitutes 'lunacy' you may be in the wrong profession. If you are doing bit tests in an int extremely frequently you might want to think about whether you are prematurely optimizing to save memory.

Even the most experienced programmers have problems with an implicit conversion to boolean. I for one appreciate this little feature.

Some programming languages do no automatic coercion at all. An integer, for example, can only be compared to another integer; assignment to a non-integer variable results in an error. Such is the hallmark of a strongly-typed language.
That Java does any coercion is a convenience for you and breaks the strong-typing model.
Mapping the entire range of integers -- or the even larger range of floats -- onto the two boolean values is fraught with disagreement over arbitrary assignment of "truthness" and "falseness".
What values map onto false and true? If you're C, only zero maps to false and all other values are true. If you're the bash shell, it's reversed.
How should negative values be mapped?
When you try to automatically convert a double to an integer, Java flags this as a "loss of precision" error. By analogy, converting a number to a boolean should also result in a loss of precision. Instead, Java chose to not syntactically support it.

What is the difference between Convert.ToInt32 and (int)?

The following code throws an compile-time error like
Cannot convert type 'string' to 'int'
string name = Session["name1"].ToString();
int i = (int)name;
whereas the code below compiles and executes successfully:
string name = Session["name1"].ToString();
int i = Convert.ToInt32(name);
I would like to know:
Why does the the first code generate a compile-time error?
What's the difference between the 2 code snippets?

(int)foo is simply a cast to the Int32 (int in C#) type. This is built into the CLR and requires that foo be a numeric variable (e.g. float, long, etc.) In this sense, it is very similar to a cast in C.
Convert.ToInt32 is designed to be a general conversion function. It does a good deal more than casting; namely, it can convert from any primitive type to a int (most notably, parsing a string). You can see the full list of overloads for this method here on MSDN.
And as Stefan Steiger mentions in a comment:
Also, note that on a numerical level, (int) foo truncates foo (ifoo = Math.Floor(foo)), while Convert.ToInt32(foo) uses half to even rounding (rounds x.5 to the nearest EVEN integer, meaning ifoo = Math.Round(foo)). The result is thus not just implementation-wise, but also numerically not the same.

(this line relates to a question that was merged) You should never use (int)someString - that will never work (and the compiler won't let you).
However, int int.Parse(string) and bool int.TryParse(string, out int) (and their various overloads) are fair game.
Personally, I mainly only use Convert when I'm dealing with reflection, so for me the choice is Parse and TryParse. The first is when I expect the value to be a valid integer, and want it to throw an exception otherwise. The second is when I want to check if it is a valid integer - I can then decide what to do when it is/isn't.

To quote from this Eric Lippert article:
Cast means two contradictory things: "check to see if this object really is of this type, throw if it is not" and "this object is not of the given type; find me an equivalent value that belongs to the given type".
So what you were trying to do in 1.) is assert that yes a String is an Int. But that assertion fails since String is not an int.
The reason 2.) succeeds is because Convert.ToInt32() parses the string and returns an int. It can still fail, for example:
Convert.ToInt32("Hello");
Would result in an Argument exception.
To sum up, converting from a String to an Int is a framework concern, not something implicit in the .Net type system.

A string cannot be cast to an int through explicit casting. It must be converted using int.Parse.
Convert.ToInt32 basically wraps this method:
public static int ToInt32(string value)
{
if (value == null)
{
return 0;
}
return int.Parse(value, CultureInfo.CurrentCulture);
}

You're talking about a C# casting operation vs .NET Conversion utilities
C# Language-level casting uses parenthesis - e.g. (int) - and conversion support for it is limited, relying on implicit compatibility between the types, or explicitly defined instructions by the developer via conversion operators.
Many conversion methods exist in the .NET Framework, e.g. System.Convert, to allow conversion between same or disparate data types.
(Casting) syntax works on numeric data types, and also on "compatible" data types. Compatible means data types for which there is a relationship established through inheritance (i.e. base/derived classes) or through implementation (i.e. interfaces).
Casting can also work between disparate data types that have conversion operators defined.
The System.Convert class on the other hand is one of many available mechanisms to convert things in the general sense; it contains logic to convert between disparate, known, data types that can be logically changed from one form into another.
Conversion even covers some of the same ground as casting by allowing conversion between similar data types.
Remember that the C# language has its own way of doing some things.
And the underlying .NET Framework has its own way of doing things, apart from any programming language.
(Sometimes they overlap in their intentions.)
Think of casting as a C# language-level feature that is more limited in nature, and conversion via the System.Convert class as one of many available mechanisms in the .NET framework to convert values between different kinds.

There is not a default cast from string to int in .NET. You can use int.Parse() or int.TryParse() to do this. Or, as you have done, you can use Convert.ToInt32().
However, in your example, why do a ToString() and then convert it back to an int at all? You could simply store the int in Session and retrieve it as follows:
int i = Session["name1"];

Just a brief extra: in different circumstances (e.g. if you're converting a double, &c to an Int32) you might also want to worry about rounding when choosing between these two. Convert.Int32 will use banker's rounding (MSDN); (int) will just truncate to an integer.

1) C# is type safe language and doesn't allow you to assign string to number
2) second case parses the string to new variable.
In your case if the Session is ASP.NET session than you don't have to store string there and convert it back when retrieving
int iVal = 5;
Session[Name1] = 5;
int iVal1 = (int)Session[Name1];

This is already discussed but I want to share a dotnetfiddle.
If you are dealing with arithmetic operations and using float, decimal, double and so on, you should better use Convert.ToInt32().
using System;
public class Program
{
public static void Main()
{
double cost = 49.501;
Console.WriteLine(Convert.ToInt32(cost));
Console.WriteLine((int)cost);
}
}
Output
50
49
https://dotnetfiddle.net/m3ddDQ

Convert.ToInt32
return int.Parse(value, CultureInfo.CurrentCulture);
but (int) is type cast, so (int)"2" will not work since you cannot cast string to int. but you can parse it like Convert.ToInt32 do

The difference is that the first snippet is a cast and the second is a convert. Although, I think perhaps the compiler error is providing more confusion here because of the wording. Perhaps it would be better if it said "Cannot cast type 'string' to 'int'.

This is old, but another difference is that (int) doesn't round out the numbers in case you have a double ej: 5.7 the ouput using (int) will be 5 and if you use Convert.ToInt() the number will be round out to 6.