Is it safe for multiple threads to read a static object?

Question

None of the threads will be modifying the variable. They are all reading. Is this safe?

Answer 1

It depends on what resources are being shared by your threads. Consider the following example:

public class MyClass
{
    private int i;
    public int Foo { get { return i++; } } 
}

public static class SharedResources
{
    public static const string SharedString;
    public static readonly MyClass SharedMyClass;
}

In this code sample, the reference to SharedString is thread-safe, because strings are read-only in C#. However, any thread may call SharedResources.SharedMyClass.Foo at any time, and because increments are not thread safe (unless using Interlocked.Increment), threads that read this property may get inconsistent results.

In short, if a class whose public API is read-only, but whose internals are not thread-safe, it's not safe simply have multiple threads reading from it.

Answer 2

In order for concurrency bugs to surface, at least one access has to be a writing/modifying one.

Be careful not to mistake getters for a variable. They might have implications not known to you when accessing.