.Net is known for its array covariance. That means that any array can be cast to an array of base elements:

public class T: B
{
}

T[] tlist = ...
B[] blist = tlist;

This feature comes at cost:

B b = ...
T t = ...

blist[0] = b; // This efficiently is: blist[0] = (T)b;
tlist[0] = t; // This is the same: tlist[0] = (T)t;

We pay the cost of additional cast, just for nothing. Let this dubious design decision opresses .Net/Java inventors.

You can eliminate the cast. Just use array of structs:

struct S<T>
{
  public T t;
}

S<T>[] slist = ...

slist[0].t = t; // Works without cast.

Measurment show that S[] is ~35% faster than T[] on write, and slower (JIT could do better) on read.

Well, ugly workaround of ugly design.

P.S. In java there is no relief...