# Fields of a struct
**URL:** https://discourse.julialang.org/t/fields-of-a-struct/107664
**Category:** General Usage
**Tags:** question
**Created:** [December 15, 2023, 1:10pm UTC](https://discourse.julialang.org/t/fields-of-a-struct/107664 "2023-12-15T13:10:48Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@jiang\_ming\_zhang](https://discourse.julialang.org/u/jiang_ming_zhang)
#### Post date: [December 15, 2023, 1:10pm UTC](https://discourse.julialang.org/t/fields-of-a-struct/107664/1 "2023-12-15T13:10:48Z")
```julia
julia> fieldnames(typeof(state))
(:hamiltonian, :replicas, :maxlength, :r_strat, :s_strat, :τ_strat, :post_step, :replica)
julia> state.step
5000
julia> state.laststep
5000
```
It is strange that while the variable ‘state’ as a datatype has no field name ‘step’, it does has a value for ‘state.step’.
What is wrong?
---
### Author:  [@fatteneder](https://discourse.julialang.org/u/fatteneder)
#### Post date: [December 15, 2023, 1:16pm UTC](https://discourse.julialang.org/t/fields-of-a-struct/107664/2 "2023-12-15T13:16:01Z")
Take a look at `propertynames`. Note that it expects an instance of a type and not a type.
---
### Author:  [@jiang\_ming\_zhang](https://discourse.julialang.org/u/jiang_ming_zhang)
#### Post date: [December 15, 2023, 1:38pm UTC](https://discourse.julialang.org/t/fields-of-a-struct/107664/3 "2023-12-15T13:38:17Z")
```julia
julia> propertynames(state)
(:hamiltonian, :replicas, :maxlength, :r_strat, :s_strat, :τ_strat, :post_step, :replica)
julia> fieldnames(typeof(state))
(:hamiltonian, :replicas, :maxlength, :r_strat, :s_strat, :τ_strat, :post_step, :replica)
```
---
### Author:  [@Sukera](https://discourse.julialang.org/u/Sukera)
#### Post date: [December 15, 2023, 1:41pm UTC](https://discourse.julialang.org/t/fields-of-a-struct/107664/4 "2023-12-15T13:41:25Z")
Seems like whoever defined the type that `state` has overloaded `getproperty`, but didn’t make that known through `propertynames` (which they should). Open an issue at the package repo in question.
---
### Author:  [@jiang\_ming\_zhang](https://discourse.julialang.org/u/jiang_ming_zhang)
#### Post date: [December 15, 2023, 1:42pm UTC](https://discourse.julialang.org/t/fields-of-a-struct/107664/5 "2023-12-15T13:42:54Z")
```julia
# Allow setting step, laststep, dτ, shift from QMCState.
function Base.getproperty(state::QMCState, key::Symbol)
if key == :step
step = state.replicas[1].params.step
return step
elseif key == :laststep
laststep = state.replicas[1].params.laststep
return laststep
elseif key == :maxlength
return getfield(state, :maxlength)[]
elseif key == :dτ
return state.replicas[1].params.dτ
elseif key == :shift
return state.replicas[1].params.shift
else
return getfield(state, key)
end
end
function Base.setproperty!(state::QMCState, key::Symbol, value)
if key == :step
for r in state.replicas
r.params.step = value
end
return value
elseif key == :laststep
for r in state.replicas
r.params.laststep = value
end
return value
elseif key == :dτ
for r in state.replicas
r.params.dτ = value
end
return value
elseif key == :shift
for r in state.replicas
r.params.shift = value
end
return value
elseif key == :maxlength
getfield(state, :maxlength)[] = value
return value
else
# This will error
return setfield!(state, key, value)
end
end
```
---
### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [December 15, 2023, 2:03pm UTC](https://discourse.julialang.org/t/fields-of-a-struct/107664/6 "2023-12-15T14:03:27Z")
you should also have:
```julia
Base.propertynames(::QMCState) = (:step, :laststep, :maxlength, :dr, :shift)
```