Why do nullable bools not allow if(nullable) but do allow if(nullable == true)?

Question

This code compiles:

private static void Main(string[] args) {     bool? fred = true;      if (fred == true)         Console.WriteLine("fred is true");     else if (fred == false)          Console.WriteLine("fred is false");     else Console.WriteLine("fred is null"); } 

This code does not compile.

private static void Main(string[] args) {     bool? fred = true;      if (fred)         Console.WriteLine("fred is true");     else if (!fred)          Console.WriteLine("fred is false");     else Console.WriteLine("fred is null"); } 

I thought if(booleanExpression == true) was supposed to be a redundancy. Why isn't it in this case?

Answer 1

There's no implicit conversion from Nullable<bool> to bool. There is an implicit conversion from bool to Nullable<bool> and that's what happens (in language terms) to each of the bool constants in the first version. The bool operator==(Nullable<bool>, Nullable<bool> operator is then applied. (This isn't quite the same as other lifted operators - the result is just bool, not Nullable<bool>.)

In other words, the expression 'fred == false' is of type bool, whereas the expression 'fred' is of type Nullable<bool> hence you can't use it as the "if" expression.

EDIT: To answer the comments, there's never an implicit conversion from Nullable<T> to T and for good reason - implicit conversions shouldn't throw exceptions, and unless you want null to be implicitly converted to default(T) there's not a lot else that could be done.

Also, if there were implicit conversions both ways round, an expression like "nullable + nonNullable" would be very confusing (for types that support +, like int). Both +(T?, T?) and +(T, T) would be available, depending on which operand were converted - but the results could be very different!

I'm 100% behind the decision to only have an explicit conversion from Nullable<T> to T.

Answer 2

Because fred is not a boolean. it is a struct, which has a boolean property called IsNull, or HasValue, or whatever... The object named fred is the complex composite object containing a boolean and a value, not a primitive boolean itself...

Below, for example is how a Nullable Int could be implemented. The generic Nullable is almost certainly implemented similarly (but generically). You can see here how the implicit and explicit conversions are implemented..

public struct DBInt    {        // The Null member represents an unknown DBInt value.        public static readonly DBInt Null = new DBInt();        // When the defined field is true, this DBInt represents a known value        // which is stored in the value field. When the defined field is false,        // this DBInt represents an unknown value, and the value field is 0.        int value;        bool defined;        // Private instance constructor. Creates a DBInt with a known value.        DBInt(int value)         {               this.value = value;               this.defined = true;        }        // The IsNull property is true if this DBInt represents an unknown value.        public bool IsNull { get { return !defined; } }        // The Value property is the known value of this DBInt, or 0 if this        // DBInt represents an unknown value.        public int Value { get { return value; } }        // Implicit conversion from int to DBInt.        public static implicit operator DBInt(int x)         { return new DBInt(x); }         // Explicit conversion from DBInt to int. Throws an exception if the        // given DBInt represents an unknown value.        public static explicit operator int(DBInt x)         {               if (!x.defined) throw new InvalidOperationException();               return x.value;        }        public static DBInt operator +(DBInt x)         { return x; }        public static DBInt operator -(DBInt x)         { return x.defined? -x.value: Null; }        public static DBInt operator +(DBInt x, DBInt y)         {               return x.defined && y.defined?                        x.value + y.value: Null;        }        public static DBInt operator -(DBInt x, DBInt y)         {               return x.defined && y.defined?                         x.value - y.value: Null;        }        public static DBInt operator *(DBInt x, DBInt y)         {               return x.defined && y.defined?                         x.value * y.value: Null;        }        public static DBInt operator /(DBInt x, DBInt y)         {               return x.defined && y.defined?                        x.value / y.value: Null;        }        public static DBInt operator %(DBInt x, DBInt y)         {               return x.defined && y.defined?                         x.value % y.value: Null;        }        public static DBBool operator ==(DBInt x, DBInt y)         {               return x.defined && y.defined?                        x.value == y.value: DBBool.Null;        }        public static DBBool operator !=(DBInt x, DBInt y)         {               return x.defined && y.defined?                        x.value != y.value: DBBool.Null;        }        public static DBBool operator >(DBInt x, DBInt y)         {               return x.defined && y.defined?                        x.value > y.value: DBBool.Null;        }        public static DBBool operator <(DBInt x, DBInt y)         {               return x.defined && y.defined?                        x.value < y.value: DBBool.Null;        }        public static DBBool operator >=(DBInt x, DBInt y)         {               return x.defined && y.defined?                         x.value >= y.value: DBBool.Null;        }        public static DBBool operator <=(DBInt x, DBInt y)         {               return x.defined && y.defined?                        x.value <= y.value: DBBool.Null;        }        public override bool Equals(object o)         {               try { return (bool) (this == (DBInt) o); }                catch  { return false; }        }        public override int GetHashCode()         { return (defined)? value: 0; }           public override string ToString()         { return (defined)? .ToString(): "DBInt.Null"; }       }