Step 1: Analyze the given differential equation.
The given equation is
\[
\frac{dy}{dx}=\frac{ax+b}{cy+d}
\]
For the general solution to represent a family of straight lines, the slope must be constant.
Step 2: Make the right-hand side constant.
If
\[
a=0
\]
and
\[
c=0,
\]
then the equation becomes
\[
\frac{dy}{dx}=\frac{b}{d}
\]
This is a constant slope, provided \(b\) and \(d\) are not both zero.
Hence,
\[
b^2+d^2\neq 0
\]
Step 3: Find the general solution.
Now,
\[
\frac{dy}{dx}=\frac{b}{d}
\]
Integrating,
\[
y=\frac{b}{d}x+C
\]
This represents a family of straight lines.
Step 4: Final conclusion.
Therefore, the required condition is
\[
\boxed{a=c=0,\ b^2+d^2\neq 0}
\]