Find a serial port device through WMI (windows management instrumentation)
The code works well, but I want to know where I can improve it to make it more resilient and less error prone.  



A very good start would be to always use braces {} although they might be optional.  
By stacking the using blocks you can save some horizontal spacing.  
By reverting the condition of if (portList.Count != 0 &amp;&amp; matchingEntities != null) you could return early and the else would be (like now) redundant and could be removed which will save some spacing too.  
The naming of the variables could need some imporvements too, e.g the plural matchingEntities doesn't match the result of the call to First().  
Storing the result of matchingEntities["DeviceID"].ToString() into a variable will speed up things.  


Implementing the mentioned points will lead to  

string comportInfo = string.Empty;

using (var entitySearcher = new ManagementObjectSearcher("root\\CIMV2", "SELECT * FROM Win32_PnPEntity WHERE Caption LIKE '%" + SerialPortToFind + "%'"))    
using (var serialPortSearcher = new ManagementObjectSearcher("root\\WMI", "SELECT * FROM MSSerial_PortName"))
{
    //testBase.SamplerWithCancel is like a for loop with exception control, a time between interations and amount of iterations to be made. It expects a true or false value to determine whether a desired condition is met. Failing to return a true value in the specified iterations throws an exception.
    testBase.SamplerWithCancel(() =&gt;
        {
        var portList = serialPortSearcher.Get().Cast&lt;ManagementBaseObject&gt;().ToList();
        var matchingEntity = entitySearcher.Get().Cast&lt;ManagementBaseObject&gt;().First();

        if (portList.Count == 0 || matchingEntity == null)
        {
            return false;
        }

        string entity = matchingEntity["DeviceID"].ToString();

        foreach (ManagementBaseObject port in portList)
        {
            if (port["InstanceName"].ToString().Contains(entity))
            {
                comportInfo = port["PortName"].ToString();
            }
        }

        return true;

    }, "Serial port not found", 3, 1500, 500, false, false);
}