Selecting all objects or elements in Blender should be a simple matter, and it is.
To select all in Blender, press A while your mouse is in the 3D viewport or outliner. You can also go to the select menu and choose "all". To deselect all press Alt+A or go to the select menu and choose None.
To select all including hidden objects or elements, first press Alt+H to unhide all and then press A in the 3D viewport.
There are cases where everything is not selected when using this shortcut. There may also be cases where we want to select all of a subset. Like all objects in a collection or all connected faces of a mesh. Let's look at those variations as well.
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Let's first look at the object level and how this works in object mode.
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We just learned that A selects all and Alt+A deselects all. But we can also have A toggle so that we can press it once to select all and again to deselect all. Alt+A will still work for deselection even if we setup A to toggle.
To setup A to toggle select all, go to Edit->Preferences and into the keymap section. Here you will find a preference window at the top where you can check "Select all toggles".
In order for an object to be selectable, it needs to be both visible and enabled for selection. By default, all objects have both these settings enabled and can therefore be selected. For example, by pressing A to select all.
There is one exception to this. That is if we are in local view for an object. You toggle local view by pressing slash on the numpad or by going to view->Local view->Toggle local view in the 3D viewport.
In local view, we cannot select other objects in the 3D viewport. But we can still select other objects if we hover our mouse over the outliner.
There are two way we can enable and disable hidden and selectable in Blender. For individual objects or collections in the outliner or for groups of object types in the "object types visibility menu". You can find this in the top right corner of the 3D viewport.
Here all types of objects are both visible as indicated by the eye icon and selectable as indicated by the mouse cursor icon.
If you want a particular object type to not be selectable you can click the arrow.
For the eye icon, press it to hide all objects of that type. If an object is hidden this way, we cannot select it in the 3D viewport, but we can still select it in the outliner if the object type is still enabled for selection.
Let's now turn our attention to the outliner. Here we have a camera and an eye icon by default. But every object and collection can also be toggled on or off for selection. We just need to first make the mouse cursor icon visible in the interface.
To do that, go to the outliner and click the filter icon in the header. Then press the mouse cursor icon the make it visible in the outliner interface.
The eye and mouse cursor icons now works the same as in the "object types and visibility" menu. But here they work for individual objects or whole collections.
For collections we simply use the icons on the same row as the collection we want to toggle selection and visibility for while for individual objects we use the icons on their respective rows.
Note that visibility and selectibility settings are maintained even if we remove the icons again using the filter menu.
We can also select one or more objects and press H to hide them. This is the same as pressing the eye icon on these objects in the outliner.
To unhide all objects press Alt+H and all eye icons will be reset to visible.
For edit mode, selecting all is similar. We use A or Alt+A and if we got it setup as described above, we toggle with just A.
Likewise, we can hide mesh by selecting it and pressing H. At that point, no operations or actions can be perforerad on those hidden elements. We can also unhide all by pressing Alt+H so that we can then select everything again.
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While in edit mode, we can still access the "objects types and visibility" menu in the 3D viewport and the outliner. But if we are already in edit mode for an object and we make it unselectable, that has no effect on the selection in edit mode.
While in edit mode, we might want to select all connected faces if our mesh is made up of multiple separate parts. In that case, select a single element in that part of the mesh and press Ctrl+L to select all linked geometry.
We can to this for faces, edges and vertices.
We can also use Shift+G in edit mode to select groups of geometry elements based on other factors, such as normals direction or vertex groups.
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Let's first look at the object level and outliner. Here we have a few options of selecting groups of objects.
We can right click a collection and choose "select objects" to select all objects in the collection. We can also right click an object that has one or more child objects and choose select hierarchy to select the parent object and all its child objects.
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We can also hold Ctrl and click objects to select multiple of them or hold Shift while clicking objects to select a range.
In the 3D viewport, the primary way of selecting groups of objects is to press Shift+G for the select grouped menu. To use it, first select an object, then press Shift+G then select the similarity you want to select other objects by.
Here we can also select by collection and children, but also other group types such as light type, keying set or by the type of object.
We can combine this with hiding objects we don't want selected to create an even smaller subset.
Another menu we can use to select groups are the select linked menu. Access it with Shift+L
Here we can select all objects with the same material attached, or with the same object data or common particle systems for example.
We started with something that seem basic, to select all. From there we fleshed out the exeptions and alternatives to selecting all and what might prevent us from selecting something. We looked at both object mode and edit mode and also selectin all in a group. like selecting all objects in a collection.
I hope this was helpful to you.
Thanks for your time.