SFDC.SalesforceApi 1.0.0.3

Code to Create Matadata Custom Fields:

@using SFDC

    SalesForceApi api = new SalesForceApi();
    var loginInfo = api.Login("abc@m.com", "********");

    ElementModel model = new ElementModel()
    {
        FullName = "Account.CustomFieldTest3__c",
        Type = FieldType.Url,
        Label = "CustomTest",
    };

    MetadataApi meta = new MetadataApi(loginInfo.sessionId,loginInfo.metadataServerUrl);
    UpsertResult[] result = meta.CreateCustomField(model);

Custom Objects :

    CustomObject co = new CustomObject();
    co.fullName = model.FullName.Replace(' ', '_') + "__c";
    co.deploymentStatus = DeploymentStatus.Deployed;
    co.deploymentStatusSpecified = true;
    co.description = model.Description;
    co.enableActivities = true;
    co.label = model.Label;
    co.pluralLabel = model.Label + "s";
    co.sharingModel = SharingModel.ReadWrite;
    co.sharingModelSpecified = true;

    co.fields = new CustomField[]{};
    co.nameField = new CustomField();
    co.nameField.label = model.Label;
    co.nameField.type = FieldType.Text;
    co.nameField.typeSpecified = true;
    UpsertResult[] result2 = meta.UpsertCustomObject(co);

Create/Update an actual Opportunity/Contact

   SalesForceApi api2 = new SalesForceApi("endPointUrl","sessionId");


    CreateSObject opportunity = new CreateSObject()
    {
        Type = "Opportunity",
        Fields = new List<ElementField>()
        {
            new ElementField()
            {
                Key = "Name",
                Value = "My New Opportunity Name"
            },
            new ElementField()
            {
                Key = "Amount",
                Value = "999"
            },
        }
    };

    SFDC.soapApi.SaveResult[] saveResults = api2.CreateElement(opportunity);


    UpdateSObject contact = new UpdateSObject()
    {
        Id = "0031W0000213yCzQAI",
        Type = "Contact",
        Fields = new List<ElementField>()
        {
            new ElementField()
            {
                Key = "FirstName",
                Value = "Otto"
            },
            new ElementField()
            {
                Key = "LastName",
                Value = "Jespersen"
            },
        }
    };

    SFDC.soapApi.SaveResult[] saveResults = api2.UpdateElement(contact);