Closes #399

src/types/TransferFunction.jl

@@ -1,12 +1,10 @@
 struct TransferFunction{TE, S<:SisoTf{T} where T} <: LTISystem{TE}
     matrix::Matrix{S}
     timeevol::TE
-    nu::Int
-    ny::Int
     function TransferFunction{TE,S}(matrix::Matrix{S}, timeevol::TE) where {S,TE}
         # Validate size of input and output names
         ny, nu = size(matrix)
-        return new{TE,S}(matrix, timeevol, nu, ny)
+        return new{TE,S}(matrix, timeevol)
     end
 end
 function TransferFunction(matrix::Matrix{S}, timeevol::TE) where {TE<:TimeEvolution, T<:Number, S<:SisoTf{T}}
@@ -35,6 +33,22 @@ Base.size(G::TransferFunction) = size(G.matrix)
 Base.eltype(::Type{S}) where {S<:TransferFunction} = S
 Base.zero(G::TransferFunction{TE,S}) where {TE,S} = tf(zeros(numeric_type(S), size(G)), G.timeevol) # can not create a zero of a discrete system from the type alone, the sampletime is not stored.
 
+function Base.getproperty(G::TransferFunction, s::Symbol)
+    s ∈ fieldnames(typeof(G)) && return getfield(G, s)
+    if s === :ny
+        return size(G, 1)
+    elseif s === :nu
+        return size(G, 2)
+    elseif s === :Ts
+        if isdiscrete(G)
+            return timeevol(G).Ts
+        else
+            @warn "Getting time 0.0 for non-discrete systems is deprecated. Check `isdiscrete` before trying to access time."
+            return 0.0
+        end
+        throw(ArgumentError("$(typeof(G)) has no property named $s"))
+    end
+end
 
 function Base.getindex(G::TransferFunction{TE,S}, inds...) where {TE,S<:SisoTf}
     if size(inds, 1) != 2