How to override ExecuteQueryAsync method?

[kgiddu]

I am trying to support Oracle database and I cannot seem to be table to override the ExecuteQueryAsync method in OracleQueryExecutor. cs

Class definition: public class OracleQueryExecutor : QueryExecutor

    public override async Task<TResult?> ExecuteQueryAsync<TResult>(
        string sqltext,
        IDictionary<string, DbConnectionParam> parameters,
        Func<DbDataReader, List<string>?, Task<TResult>>? dataReaderHandler,
        string dataSourceName,
        HttpContext? httpContext = null,
        List<string>? args = null)
    {
       // Execute Oracle specific queries if not call base. 
       
        TResult? result = await base.ExecuteQueryAsync(
            sqltext,
            parameters,
            dataReaderHandler,
            dataSourceName,
            httpContext,
            args);

        return result;
    }

I get this error
'OracleQueryExecutor.ExecuteQueryAsync(string, IDictionary<string, DbConnectionParam>, Func<DbDataReader, List?, Task>?, string, HttpContext?, List?)': return type must be 'Task<TResult?>' to match overridden member 'QueryExecutor.ExecuteQueryAsync(string, IDictionary<string, DbConnectionParam>, Func<DbDataReader, List?, Task>?, string, HttpContext?, List?)'

Tried this as well
public override async Task<TResult?> ExecuteQueryAsync(
string sqltext,
IDictionary<string, DbConnectionParam> parameters,
Func<DbDataReader, List?, Task>? dataReaderHandler,
string dataSourceName,
HttpContext? httpContext = null,
List? args = null)
{
TResult? result = default(TResult);

        return result;
    }

Got this error
Cannot implicitly convert type 'TResult' to 'TResult?'. An explicit conversion exists (are you missing a cast?)

[JerryNixon]

Two problems exist in your code.

First problem: QueryExecutor is generic. Your class must inherit the closed generic type.

Wrong

public class OracleQueryExecutor : QueryExecutor

Correct

public class OracleQueryExecutor : QueryExecutor<OracleConnection>

Without the generic argument the compiler cannot match the generic override.

Second problem: The overridden method itself is generic. The <TResult> must appear on the method name.

Wrong

public override async Task<TResult?> ExecuteQueryAsync(

Correct

public override async Task<TResult?> ExecuteQueryAsync<TResult>(

Working example

public class OracleQueryExecutor : QueryExecutor<OracleConnection>
{
    public OracleQueryExecutor(
        DbExceptionParser dbExceptionParser,
        ILogger<IQueryExecutor> logger,
        RuntimeConfigProvider configProvider,
        IHttpContextAccessor httpContextAccessor,
        HotReloadEventHandler<HotReloadEventArgs>? handler)
        : base(dbExceptionParser, logger, configProvider, httpContextAccessor, handler)
    {
    }

    public override async Task<TResult?> ExecuteQueryAsync<TResult>(
        string sqltext,
        IDictionary<string, DbConnectionParam> parameters,
        Func<DbDataReader, List<string>?, Task<TResult>>? dataReaderHandler,
        string dataSourceName,
        HttpContext? httpContext = null,
        List<string>? args = null)
    {
        return await base.ExecuteQueryAsync(
            sqltext,
            parameters,
            dataReaderHandler,
            dataSourceName,
            httpContext,
            args);
    }
}

Third issue that caused the second error: If nullable reference types are disabled, TResult? on an unconstrained generic can produce the compiler message you saw. Ensure the project file contains:

<PropertyGroup>
  <Nullable>enable</Nullable>
</PropertyGroup>

After those three changes the override should compile.

---

Extending DAB with a Custom Database Executor

Data API builder (DAB) natively supports SQL Server compatible databases, PostgreSQL, MySQL, and Cosmos DB.
This guide shows how to create a custom QueryExecutor for a database engine that DAB does not support out of the box (for example, Oracle).

Prerequisites

• Familiarity with C# generics and nullable reference types.
• The Oracle managed driver NuGet package
  https://www.nuget.org/packages/Oracle.ManagedDataAccess.Core
  (or the equivalent driver for your database).

---

1. Project setup

Create a new class-library project and reference the Azure.DataApiBuilder.Core package.

In your .csproj, enable nullable reference types. The QueryExecutor<TConnection> base class uses TResult? on unconstrained generic type parameters.

<PropertyGroup>
  <Nullable>enable</Nullable>
</PropertyGroup>

---

2. Implement OracleQueryExecutor

Inherit from QueryExecutor<TConnection> and supply your provider’s concrete connection type as the generic argument.

using System.Data.Common;
using Azure.DataApiBuilder.Core.Configurations;
using Azure.DataApiBuilder.Core.Models;
using Azure.DataApiBuilder.Core.Resolvers;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Logging;
using Oracle.ManagedDataAccess.Client;

public class OracleQueryExecutor : QueryExecutor<OracleConnection>
{
    public OracleQueryExecutor(
        DbExceptionParser dbExceptionParser,
        ILogger<IQueryExecutor> logger,
        RuntimeConfigProvider configProvider,
        IHttpContextAccessor httpContextAccessor,
        HotReloadEventHandler<HotReloadEventArgs>? handler)
        : base(dbExceptionParser, logger, configProvider, httpContextAccessor, handler)
    {
    }

    // Override only the methods that need Oracle-specific behavior.
    // Other virtual methods (CreateConnection, PrepareDbCommand, etc.)
    // fall back to the base-class implementations.
    //
    // For Oracle you will commonly override PrepareDbCommand(...) or
    // PopulateDbTypeForParameter(...) to use OracleParameter and set
    // OracleParameter.OracleDbType.
    public override async Task<TResult?> ExecuteQueryAsync<TResult>(
        string sqltext,
        IDictionary<string, DbConnectionParam> parameters,
        Func<DbDataReader, List<string>?, Task<TResult>>? dataReaderHandler,
        string dataSourceName,
        HttpContext? httpContext = null,
        List<string>? args = null)
    {
        return await base.ExecuteQueryAsync(
            sqltext,
            parameters,
            dataReaderHandler,
            dataSourceName,
            httpContext,
            args);
    }
}

---

3. Common pitfalls

Missing generic type argument

// WRONG — QueryExecutor without a type argument is an open generic type
public class OracleQueryExecutor : QueryExecutor

// CORRECT
public class OracleQueryExecutor : QueryExecutor<OracleConnection>

Missing <TResult> on the override

// WRONG
public override async Task<TResult?> ExecuteQueryAsync(

// CORRECT
public override async Task<TResult?> ExecuteQueryAsync<TResult>(

Nullable reference types disabled

Without nullable annotations enabled, the TResult? signature used by the base class can cause confusing compiler diagnostics.

Ensure the project includes:

<PropertyGroup>
  <Nullable>enable</Nullable>
</PropertyGroup>

---

4. Registering the custom executor

Register the executor in the dependency injection container during startup.

services.AddSingleton<IQueryExecutor, OracleQueryExecutor>();

If your host uses multiple database providers, the runtime must also map the configured data source type to the appropriate executor implementation.

---

5. Supported override points

QueryExecutor<TConnection> exposes several virtual members you can override.

Member	Description
CreateConnection(string)	Create and return a new database connection.
ExecuteQueryAsync<TResult>(...)	Async query execution with retry.
ExecuteQuery<TResult>(...)	Sync query execution with retry.
ExecuteQueryAgainstDbAsync<TResult>(...)	Async execution against an open connection.
ExecuteQueryAgainstDb<TResult>(...)	Sync execution against an open connection.
PrepareDbCommand(...)	Build the DbCommand from SQL text and parameters.
GetSessionParamsQuery(...)	Return optional session-context SQL to prepend.
PopulateDbTypeForParameter(...)	Set the DbType on a DbParameter.
SetManagedIdentityAccessTokenIfAnyAsync(...)	Attach a Managed Identity token to the connection.
SetManagedIdentityAccessTokenIfAny(...)	Sync variant of the above.
GetMultipleResultSetsIfAnyAsync(...)	Process multiple result sets from a reader.
ConnectionStringBuilders	Dictionary mapping data-source names to connection string builders.