您应该将这个JSON反序列化为类，然后对类对象执行任何您想要的操作。
您可以使用NewtonSoft.Json dll实现此目的。

public class LiquiditySource
{
    public int ID { get; set; }
    public object ParentID { get; set; }
    public string Name { get; set; }
    public object SequenceNo { get; set; }
    public string Caption { get; set; }
    public string ConnectivityInfo { get; set; }
}

public class LookupData
{
    public List<LiquiditySource> LiquiditySources { get; set; }
}

public class SampleClass
{
    public string AccessToken { get; set; }
    public int LoggedInUserID { get; set; }
    public LookupData LookupData { get; set; }
}

string json = "your json string";
SampleClass obj = JsonConvert.DeserializeObject<SampleClass>(json);

您可以获得如下属性：

List<LiquiditySource> ls =  obj.LookupData.LiquiditySources;
    foreach(LiquiditySource liquiditySource in ls)
    {
        string connectivityInfo = liquiditySource.ConnectivityInfo;

    }

如果只想获取所有ConnectivityInfo值，也可以使用LINQ。

List<string> lsConnectivityInfo = obj.LookupData.LiquiditySources.Select(c => c.ConnectivityInfo).ToList();

如果你想进一步将connectivityInfo字符串反序列化为对象并获取它的属性，那么你必须再次将它反序列化为对象。您还必须修改JSON。
修改的JSON：

{
    "AccessToken": "1929c63a-9c72-4c0d-aea8-345c0b5411e9",
    "LoggedInUserID": 1,
    "LookupData": {
        "LiquiditySources": [{
            "ID": 1,
            "ParentID": null,
            "Name": "testname1",
            "SequenceNo": null,
            "Caption": "test1",
            "ConnectivityInfo": "{\"Port\":11111,\"Username\":\"testuser1\",\"Password\":\"password1\"}"
        }, {
            "ID": 2,
            "ParentID": null,
            "Name": "testname2",
            "SequenceNo": null,
            "Caption": "test2",
            "ConnectivityInfo": "{\"Port\":22222,\"Username\":\"testuser2\",\"Password\":\"password2\"}"
        }]
    }
}

再上一堂课：

public class ConnectivityInfo
{
    public int Port { get; set; }
    public string Username { get; set; }
    public string Password { get; set; }
}

然后执行以下操作：

foreach (LiquiditySource liquiditySource in ls)
    {
        string connectivityInfo = liquiditySource.ConnectivityInfo;
        ConnectivityInfo connectivityInfoObj = JsonConvert.DeserializeObject<ConnectivityInfo>(connectivityInfo);

    }