Converting a Predicate<T> to a Func<T, bool>

The two types represent the same logical signature, but that doesn't mean they're just interchangable. A straight assignment won't work, for example - but you can create a new Func<T, bool> from the Predicate<T, bool>. Sample code:

Predicate<string> pred = x => x.Length > 10;
// Func<string, bool> func = pred; // Error
Func<string, bool> func = new Func<string, bool>(pred); // Okay

This is a bit like having two enum types with the same values - you can convert between them, but you have to do so explicitly. They're still separate types.

In your case, this means you could write:

public bool DoAllHaveSomeProperty()
{
    return m_instrumentList.All(new Func<T, bool>(m_filterExpression));
}

The lambda expression approach suggested by other answers will work too, of course.

public bool DoAllHaveSomeProperty()
{
    return m_instrumentList.All(i => m_filterExpression(i));
}

You can convert a predicate to a method by calling Invoke. All delegates have this member. Delegates don't have structural identity, but methods can be converted to matching delegates. This fix has a minor performance cost, as it adds an extra layer of indirection. However, most solutions to this problem have that problem. Eric Lippert discusses this in more detail at https://web.archive.org/web/20140625132124/http://blog.coverity.com/2014/06/18/delegates-structural-identity/ .

In your specific case, replace return m_instrumentList.All(m_filterExpression); with return m_instrumentList.All(m_filterExpression.Invoke);

Sample code demonstrating the actual problem.

void Main()
{
    Predicate<int> t1 = Foo;
    Func<int,bool> t2 = Foo;
    Predicate<int> t3 = t2.Invoke; //Legal
    Func<int,bool> t4 = t1.Invoke; //Legal
    Predicate<int> t5 = t2; //Illegal
    Func<int,bool> t6 = t1; //Illegal
}

bool Foo(int x)
{
    return x > 20;
}