Implementing an inteface

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C#

.NET

inheritance

I'm confused about the implementation of interfaces.

ICollection<T> has the property IsReadOnly according to MSDN[^]
-And-
Collection<T> impliments ICollection<T> according to MSDN[^]
-So-
I thought that Collection<T> would have the property IsReadOnly.
-However-

Collection<string> testCollection = new Collection<string>();
Console.WriteLine(testCollection.IsReadOnly);

The above code gives the compiler error:
'System.Collections.ObjectModel.Collection<string>' does not contain a definition for 'IsReadOnly' and no extension method 'IsReadOnly' accepting a first argument of type 'System.Collections.ObjectModel.Collection<string>' could be found (are you missing a using directive or an assembly reference?)
-While-

Collection<string> testInterface = new Collection<string>();
Console.WriteLine(((ICollection<string>)testInterface).IsReadOnly);

The above code works.
-Question-
I thought classes implementing interfaces had to implement every property, so why doesn't testCollection have the IsReadOnly property unless you cast it as ICollection<string>?