You can modify the parameters of an effect, in some cases even while the effect is playing. Do this by using the IDirectInputEffect::SetParameters method.

The dwDynamicParams member of the DIEFFECTINFO structure tells you which effect parameters can be changed while an effect is playing. If you attempt to modify an effect parameter that cannot be modified while the effect is playing, and the effect is still playing, Microsoft® DirectInput® normally stops the effect, updates the parameters, and restarts the effect. You can override this default behavior by passing the DIEP_NORESTART flag.

The following code example changes the magnitude of the constant force that was set in the example under Creating an Effect.

DIEFFECT        diEffect;           // Parameters for effect
DICONSTANTFORCE diConstantForce;    // Type-specific parameters
 
diConstantForce.lMagnitude = 5000;
diEffect.dwSize = sizeof(DIEFFECT); 
diEffect.cbTypeSpecificParams = sizeof(DICONSTANTFORCE); 
diEffect.lpvTypeSpecificParams = &diConstantForce;
hr = lpdiEffect->SetParameters(&diEffect, DIEP_TYPESPECIFICPARAMS);

The DIEP_TYPESPECIFICPARAMS flag ensures that the transfer of data from the DIEFFECT structure is restricted to the relevant members, so that you do not have to initialize the entire structure and so that the minimum possible amount of data needs to be sent to the device.