The following code throws an IndexOutOfRangeexception:
var builder = ImmutableArray.CreateBuilder<Color>(5);
builder[3] = Colors.Red;
I expected this would work, as the documentation says the following:
ImmutableArray<T>.CreateBuilder<T>(Int32)Creates a mutable array that can be converted to an ImmutableArray without allocating new memory; Int32 = The initial capacity of the builder. returns aBuilder
Builderclass: A writable array accessor that can be converted into an ImmutableArray instance without allocating extra memory.
Builder[int32]Gets or sets the item at the specified index.
Nothing in the docs says this would be an invalid expectation.
The same code for a basic array passes:
var colors = new Color[5];
colors[3] = Colors.Red;
I expected to get "a mutable array" by using a Builder but a regular mutable array behaves differently than the Builder.
For my app’s purpose, indices are keys, and I need to be able to set the item at a given index just like a basic array.
Edit:
This works:
var arr = new Color[5];
arr[3] = Colors.Red;
var array = arr.ToImmutableArray();
This doesn’t:
var builder = ImmutableArray.CreateBuilder<Color>(5);
builder[3] = Colors.Red;
var array = builder.ToImmutable();
For my question, I can accomplish it via the above but take the penalty of additional allocation.
>Solution :
ImmutableArray<T>.Builder has the same semantics as List<T>. Therefore, creating one with a count actually gives you one with a specified capacity and a count of 0. And just like with a list, accessing elements above the count is an error.
Instead, set Count directly to forcibly resize the array, filling it with default values:
var builder = ImmutableArray.CreateBuilder<Color>();
builder.Count = 5; // this will increase capacity if needed
builder[3] = Colors.Red; // no more errors!