Why can’t I define a default constructor for a struct in .NET?

In .NET, a value type (C# struct) can’t have a constructor with no parameters. According to this post this is mandated by the CLI specification. What happens is that for every value-type a default constructor is created (by the compiler?) which initialized all members to zero (or null).

Why is it disallowed to define such a default constructor?

One trivial use is for rational numbers:

public struct Rational {
    private long numerator;
    private long denominator;

    public Rational(long num, long denom)
    { /* Todo: Find GCD etc. */ }

    public Rational(long num)
    {
        numerator = num;
        denominator = 1;
    }

    public Rational() // This is not allowed
    {
        numerator = 0;
        denominator = 1;
    }
}

Using current version of C#, a default Rational is 0/0 which is not so cool.

PS: Will default parameters help solve this for C# 4.0 or will the CLR-defined default constructor be called?

Jon Skeet answered:

To use your example, what would you want to happen when someone did:

 Rational[] fractions = new Rational[1000];

Should it run through your constructor 1000 times?

Sure it should, that’s why I wrote the default constructor in the first place. The CLR should use the default zeroing constructor when no explicit default constructor is defined; that way you only pay for what you use. Then if I want a container of 1000 non-default Rationals (and want to optimize away the 1000 constructions) I will use a List<Rational> rather than an array.

This reason, in my mind, is not strong enough to prevent definition of a default constructor.